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First Edition March, 1909 

Second Edition --.--. November, 1909, 

Third Edition October, 1912. 

Fourth Edition ------ August, 1920. 

Beprinted - - - -, - - - . May, 1922. 


In again revising this book I have conformed to. the 
eariier plan of writing what I should have said if 
the lectures had been delivered in 1920 instead of 
1908. The original statement has been amplified 
rather than modified. It has lost, long since, the 
appearance of challenge to existing theories which at 
first it may have presented. 

But the subject has now grown entirely beyond 
the power of being fully encompassed by the original 
very simple and popular mode of treatment. I have 
thought it best, therefore, not to compress the original 
part unduly, as it still may serve a useful purpose 
to those not familiar with scientific conceptions, but 
to add, as a second part, a more briefly written and 
less elementary account of the later developments, 
particularly those that bear upon the problem of 
the constitution of the atom. It is to be hoped 
that even those who are not chemists or physicists, 
who have followed the exposition in the first part, 
may not be entirely unable to profit by the second 
part. Though, naturally, the new subject-matter, 
by reasons of its more general and often more specu- 
lative character — much of it still being in the making 
— cannot but be more difficult to understand than the 
original work, which dealt with distinct and easily 
understood steps in the progress of knowledge, made 
once and for all time. 


The Uniy£bsitt of Oxford, 
July., 1920 


The present-day interpretation of radium, that it is 
an element undergoing spontaneous disintegration, 
was put forward in a series of joint scientific com- 
munications to the Philosophical Magazine of 1902 
and 1903 by Professor Rutherford, now of Manchester 
University, and myself. As its application is not 
confined to the physical sciences, but has a wide and 
general bearing on our whole outlook upon Nature, I 
have attempted in this book a presentation of the 
subject in non- technical language, so that the ideas 
involved, and their bearing upon current thought, may 
be within the reach of the lay reader. Although written 
in non-technical language, no effort has been spared to 
get to the root of the matter and to secure accuracy, so 
that possibly the book may prove serviceable to workers 
in other fields of science and investigation as well as 
to the general public. 

The book contains the main substance of six popular 
experimental lectures delivered in the University of 
Glasgow at the beginning of the year, but being relieved 
from the necessity, always present in lecturing, of co- 
ordinating the experimental and descriptive sides, I 
have, while adhering to the lecture form of address, 
entirely rearranged and very largely rewritten the 
subject matter in order to secure the greatest possible 
degree of continuity of treatment. Certain portions 
of the lectures, for example those dealing with the 
X-rays and the spectra of elements, have been omitted, 
and attention thereby concentrated upon radium, the 
chief topic. In addition, I have briefly embodied the 


results of important discoveries which have appeared 
since the date of the lectures, particularly the experi- 
ments of Professor Rutherford and Dr. Geiger in count- 
ing the number of a-particles expelled by radium. The 
book also contains some account of the arrangement 
by means of which I have recently succeeded in 
detecting and measuring the quantity of the helium 
generated from the common radio-elements uranium 
and thorium. 

I have borrowed freely from numerous scattered 
lectures and addresses bearing on the subject which 
I have from time to time been invited to deliver, and 
may mention in particular the Wilde lecture to the 
Manchester Literary and Philosophical Society, 1904, 
the Presidential and other addresses to the Rontgen 
Society, 1906, the opening of the discussion on the 
evolution of the elements in Section A of the British 
Association Meeting in York, 1906, and the Watt 
lecture to the Greenock Philosophical Society. 1908. 

The University, Glasgow, 
November, 1908. 




Radioactivity, a new science — Its discovery — The four experi- 
mental effects of radioactivity — The rays of radioactive sub- 
stances — The continuous emission of energy from the radio- 
elements - - - - - - -1 



Radioactivity an unalterable atomic property — The radioactivity 
of thorium — Pitchblende — Quantity of radium in pitchblende — 
The smallest quantity of radium detectable — Experiments with 
radium — Cost of radium — The doctrine of energy — Measure- 
ment of the energy emitted by radium — The source of cosmical 
energy — Radium and the " physically impossible " - - 12 



The radiations of the radio-elements — a-, j3-, and y-rays — Test of 
penetrating power — Experiments with the penetrating p- and 
y-rays — The feebly penetrating a-rays — Experiment with a- 
rays — The range of a-rays in air — The physical nature of 
radiation — Corpuscular radiation — The wave theory of hght — 
a- and j3- rays due to the expulsion of particles — The individual 
atom of matter — The spinthariscope — The decay of a-radiation 
— Counting the a-particles - - - - - 28 




The |3-rays — Deviation by a magnet — Electric charge carried by 
^-rays — The nature of electricity — Radiant matter or cathode- 
rays — The electron — Inertia or mass — Velocity of the /S-rays — 
The radium clock — Magnetic deviation of the a-particle — Its 
velocity — Passage of a-particles through matter — Scattering 
of a-particles — Method of rendering the track of rays visible — 
The fates of a-particles - - - - - - 47 



The source of radioactive energy — Two alternative theories — ^The 
internal energy of matter — Radium a changing element — 
Disintegration in cascade — The successive outbursts of energy 
— The radium emanation — Experiments with the emanation — 
Its condensation by cold — The infinitesimal quantity of the 
emanation — Its radioactivity — The chemical character of the 
emanation — The heat evolved by the emanation — ^The decay 
of the emanation — Its reproduction by radium — Atomic dis- 
integration — Radioactive equilibrium — Energy of radioactive 
change — All radioactive changes equally detectable - - 68 



The connection of the a-particle with radioactive changes — Helium 
and the a-particle — The ultimate products — Discovery of 
helium, solar and terrestrial — Prediction of the production of 
helium — Production of helium from radium — Its production 
from uranium and thorium — Identity of the a-particle and 
helium — The first change of radium — Radioactive recoil - 93 



Questions of nomenclature — Definition of the atom — Elements 
and chemical compounds — The experimental facts — The nature 
of atomic disintegration — The chance of disintegration — 



The period of average life of a disintegrating atom — ^The un- 
known cause of disintegration — Determination of the period of 
average life — The period of average fife of radium — The total 
energy evolved in the complete disintegration of radium - 105 



Why has radium survived ? — The reproduction of radium — 
The ratio between the quantities of uranium and radium in all 
minerals — Hydraulic analogy to radioactive change — The age of 
pitchblende — Uranium X — Attempts to detect a growth of 
radium — Existence of intermediate products — Ionium — 
Production of radium by uranium — The stately procession of 
elementary evolution .---.. 121 



The later changes of radium — ^The active deposit of radium — The 
radiations from the active deposit — Experiments with the 
active deposit — Radium A — Radium B and C — The radiation 
from the emanation — The later slow changes of radium — 
Radium D, E, and F — Polonium — The ultimate product of 
radium — Uranium I, and Uranium II — Uranium X^, and 
Uranium X2 (Brevium) — Radium C and Radium C^ - - 136 



Ratio of quantities of polonium and radium in minerals — Table 
of the ratio of the quantities of all the products of uranium — 
Impossibility of concentrating many of the products of dis- 
integration — Increase of activity of radium with time — The 
rarity of elenients — The currency metals — The nature of atoms 
— The velocity of a-particles — StabiUty and survival of ele- 
ments — Connection between range of a-rays and period — 
Pleochroic halos — Uranium and thorium halos - - - 152 




The potentialities of matter — Why radium is unique — The total 
energy evolved by uranium — The importance of transmutation 
— Primitive man and fire — Source of cosmical energy — Radium 
in the earth's crust — Various possible fates of the earth — The 
most probable view — Radioactivity and mythology — The new 
prospect - - - - • - - 168 



The thorium disintegration series — Mesothorium and radiothorium 
— Radioactivity of thorium — Mesothorium — ^The thorium 
emanation — Radiothorium — Experiments with the thorium 
emanation — Thorium A — The actinium disintegration series — 
The origin of actinium — Multiple atomic disintegration — 
Branch series of thorium and radium — The actinium branch 
series — The actinium emanation — Actinium A — Eka-tan- 
talum or proto-actinium — Uranium Y — Table of complete dis- 
integration series — The unsolved riddle of matter - - 186 



A flood of knowledge — ^The nature of mass — Sir J. J. Thomson's 
model atom — The periodic law — Electrolytic dissociation — 
The outermost region of the atom .... 209 



The innermost region of the atom — An artificial transmutation 

— Atoms compared and contrasted with solar systems - 220 





Elements which are chemically identical — The periodic law and 
radioactive changes — The atomic number — Isotopic elements — 
The problem of the ancient alchemist - - - - 227 



The X-ray spectra of the elements — The y-rays — The intermediate 
region of atomic structure — The homogeneous characteristic 
X-rays of Barkla — ^The atomic mass or weight — The element 
lead — The separation of isotopes — Neon and Metaneon — The 
general prevalence of isotopism — The problem of transmuta- 
tion — Conclusion ..,.._ 234 

INDEX - - -253 

'o face 




1. Becquerel's uranium radiograph of an aluminium"! 

medallion - - - - - 1 

2. Welsbach mantle, taken by the rays from the | 

thorium contained in it - - - 1 

3- Photograph and radiograph of a piece of pitch- 
blende (Sir William Crookes) - . . „ 15 
4j Photograph of silk tassel electrified by friction -| 

5. The same discharged by the rays of radium - ) " 

6. Radium writing on a photographic plate - 

7. Box of compasses taken by y-rays of radium 

8. Diagram of coated flask and radium-covered dish 

for showing a-rays - - - - - - 35 

9. Photograph of the same apparatus - -To face 35 

10. Diagram of Spinthariscope of Sir William 

Crookes - - - - .--43 

11. Photograph of the Spinthariscope - - - To face 35 

12. Photograph of the electro-magnet for deviating 

the /3-rays . - . . - ,,47 

13. Diagram of magnetic deviation of /3-rays - - - - 49 

14. Diagram of Crookes' tube to show magnetic devia- 

tion of cathode-rays - - - - - - 54 

15. Diagram of Strutt's radium clock - - - - - 59 

16. Photograph of radium clock - - -To face 47 

17. Tracks of a-particles photographed by C. T. R 

Wilson ..... 

18. Track of a single a- and of a single /3-particle - /" » 65 

19. Tracks of two a-particles — one straight, one twice 

deflected - - - - 

20. Photograph of tube containing AviJIemite - -\ 

21. Photograph of the same tube by its own light when r „ 78 

containing radium emanation - - --' 

22. Diagram of apparatus for showing the condensa- 

tion of the radium emanation - - - - - 81 

23. Diagram of the first disintegration of radium - - - 94 




24. Photograph of the spectrum-tube in which the pro- 

duction of heUum from radium emanation was 
observed . . . . - 

25. Photograph by Dr. Giesel of the spectrum of 

helium produced from radium 

26. Photograph of apparatus for detecting and measur- 

ing heUum produced from uranium and thor- 
ium ------ 

27. Diagram showing the first change of radium 

28. Diagram for the first disintegration of uranium 

29. Diagram of the uranium-radium disintegration 

series (initial changes) - - - 

30. Diagram of the first four disintegrations of radium 

31. Diagram of apparatus for obtaining the active 

deposit of radium - . - . 

32. Photograph of the same apparatus 

33. Diagram of the later disintegrations of radium 

34. Diagram of the complete uranium disintegration 

series ------ 

85. Microphotograph of uranium and thorium pleo-] 
chroic halos ( Joly) - - - - 1 

36. Enlarged photograph of uranium halo showing! 

ring due to Radium A - - •) 

37. Diagram of the complete thorium disintegration 

series ------ 

38. Diagram of the complete actinium disintegration 

series ------ 

39. The brandling of the thorium series 

40. The branching of the radium series 

41. Initial part of uranium series showing branch 

actinium series . - - - 

42. Table showing complete disintegration series 

43. The periodic table of the elements - 

44. Chart showing a- and /3-change and periodic law 

generalisation .... 

To face 99 

„ 100 

- 103 

- 129 

- 133 

- 139 

- 141 

To face 141 

- 146 

- 150 

To face 166 

- 190 

- 199 

- 201 

- 202 

- 206 

- 207 

- 214 

- 230 







Radioactivity, a New Science. 

One of the main duties of science is the correlation 
of phenomena, apparently disconnected and even 
contradictory. For example, chemistry teaches us 
to regard under one aspect, as various types of 
cambustion or oxidation, the burning of a candle, 
the rusting of metals, the physiological process of 
respiration, and the explosion of gunpowder. In each 
process there is the one common fact that oxygen 
enters into new chemical combinations. Similarly 
to the physicist, the fall of the traditional apple of 
Newton, the revolution of the earth and planets round 
the sun, the apparitions of comets, and the ebb and 
flow of the tides are all phases of the universal law 
of gravitation. A race ignorant of the nature of 
combustion or of the law of gravitation, and ignorant 
of the need of such generalisations, could not be con- 
sidered to have advanced far along the paths of 
scientific discovery. The phenomena with which I 
am concerned in these lectures belong to the newly- 
born science of radioactivity and to the spontaneous 
disintegration of elements which the study of radio- 


activity has revealed to us. It is a natural inquiry 
to ask — To what most nearly are these new phe- 
nomena correlated ? Is it possible to give, by the 
help of an analogy to familiar phenomena, any correct 
idea of the nature of this new phenomenon " Radio- 
activity " ? The answer may surprise those who 
hold to the adage that there is nothing new under the 
sun. Frankly, it is not possible, because in these latest 
developments science has broken fundamentally new 
ground, and has delved one distinct step further down 
into the foundations of knowledge. 

During the century which has just closed there 
occurred, it is true, at an ever-increasing rate, a cease- 
less extension of our knowledge of the nature of matter 
upon which physical science is largely based. Yet this 
advance was for the most part an expansion rather than 
a deepening. It was concerned with what may be 
termed atomic and molecular architecture, the external 
qualities of atoms and the construction and study of 
complexes built of atoms — that is to say, molecules. 
As buildings are built of bricks, so compounds can 
nowadays be built up out of atoms. The atoms are 
to the chemist and physicist what bricks are to the 
architect — the units supplied ready-made to a certain 
limited number of standard specifications and dimen- 
sions capable of an endless variety of combinations 
and arrangements, each with its own peculiarities and 
external relationships. 

The century which has just begun has seen the first 
definite and considerable step taken into the ultimate 
nature of these units of matter or atoms, which is in one 
sense not merely an extension of existing knowledge or 
principles, but a radically new departure. Radio- 
activity is a new primary science owing allegiance 
neither to physics nor chemistry, as these sciences were 
understood before its advent, because it is concerned 
with a knowledge of the elementary atoms themselves 
of a character so fundamental and intimate that the 
old laws of physics and chemistry, concerned almost 


wholly with external relationships, do not suffice. 
This first step has indeed emphasised how superficial 
our knowledge of matter has really been. If one 
were to demonstrate to an architect that the bricks 
he habitually and properly employs in his constructions 
were under other circumstances capable of entirely 
different uses — let us say, for illustration, that they 
could with effect be employed as an explosive incom- 
parably more powerful in its activities than dynamite — 
the surprise of the architect would be no greater than 
the surprise of the chemist at the new and undreamt 
of possibilities of matter demonstrated by the mere 
existence of such an element as radium. 

In this first lecture our attention will be mainly 
directed to the one outstanding feature in connection 
with radium, and the property of radioactivity which 
it exhibits to an extraordinary degree, in which the 
whole range of its remarkable features are epitomised. 
The radioactive substances evolve a perennial supply 
of energy from year to year without stimulus and without 
exhaustion. It would be idle to deny with regard 
to this that physical science was taken completely by 
surprise. Had any one twenty-five years ago ventured 
to predict radium he would have been told simply 
that such a thing was not only wildly improbable, 
but actually opposed to all the established principles 
of the science of matter and energy. So drastic an 
innovation was, it is true, unanticipated. Radium, 
however, is an undisputed fact to-day, and there is no 
question which would have triumphed in the conflict, 
had its existence conflicted with the established prin- 
ciples of science. Natural conservatism and dislike of 
innovation appear in the ranks of science more strongly 
than most people are aware. Indeed, science is no 
exception. There was, however, never the slightest 
ground for assuming that because the new facts were 
startling and unexpected they must necessarily conflict 
with older knowledge. That would be to pay science 
a poor compliment. Some of the new facts we shall 


discuss in the lectures appeared at first, and may- 
even yet appear to you, almost incredible, but that is 
only on account of the entire newness of the whole 
region to which they belong. Into this region the 
older chemistry and physics have, as we have seen, 
never before penetrated. It is not until we begin 
to apply to the new facts the established principles of 
science, which have served so well of old, that their 
full significance gradually becomes evident. Keep 
in mind that our knowledge of Nature is always of 
necessity partial, and is bounded in all directions by 
certain inevitable but too often forgotten limitations 
connected, for example, with the briefness of human 
life and the physical impossibility of pursuing investi- 
gations except under conditions where the life of 
the investigator can be maintained. The laws and 
principles of physical science, old and new, are alike 
subject to these perpetual limitations, and are neces- 
sarily only true within these limits. From this point 
of view there is nothing in the many surprising pro- 
perties of radium which conflicts with a single estab- 
lished principle of older science. Physics and chemistry 
remain almost unchanged where they were, and radio- 
activity, so far as it is concerned with the correctness 
of their principles, has, as a matter of fact, given to 
the old laws and theories a fuller and truer signifi- 
cance than they had before. The extension of the 
old theories which has been rendered necessary has 
not been revolutionary in any destructive sense. It 
is wonderful how accommodating a true theory is to 
new truth, apparently of a diametrically opposite 
character, and this not in any sense of mere ingenuity 
of explanation, but in a manner that arrests the in- 
vestigator, and is his sign that he is on safe ground. 
It may seem a paradox, but from the first the best 
proof of the newer views, to my mind, was in the com- 
pleteness with which the strange, newly-won know- 
ledge of radioactivity harmonised with the old views of 
the chemist about atoms and elements. On the other 


hand this gratifying harmony, where conflict might 
have been expected, is not a surrender. On every 
hand new vistas of thought are opening out. We 
see the simple and direct answer to many problems 
before deemed insoluble. We recognise now causes at 
work where before we only saw effects, many of them 
so familiar and ingrained in our consciousness that the 
necessity for a cause had been almost overlooked, or, 
if felt at all, met perfunctorily and wholly inadequately 
by existing knowledge. Highly technical and compli- 
cated as many of the researches on radioactivity are, 
the main conclusions of the science are as simple and 
certain as they are fundamental, and of general interest. 
It is the duty of every educated man to make himself 
aware of the chief bearings of these conclusions, for 
they touch human life strangely at many points, and 
are destined in the future to influence profoundly the 
course of philosophic thought. In a few years the 
elementary principles of radioactivity will be taught 
in all schools as belonging to the very beginnings of 
physical science. To-day, while all is strange and new 
and the very name of the science even unfamiliar, it 
may appear a far cry to attempt to foretell the effects 
these discoveries, concerned primarily with the ultimate 
nature of matter, are destined to exert on our concep- 
tions of the ultimate destiny of man. But already 
the most direct connection is apparent. Indeed, this 
aspect of the advance is perhaps the most revolu- 
tionary. We shall be able to see more clearly at the 
end how this has come about. At present it is suffi- 
cient to indicate that radioactivity has introduced a 
new conception into the fundamental problems of 
existence. By its conclusion that there is imprisoned 
in ordinary common matter vast stores of energy, 
which ignorance alone at the present time prevents 
us from using for the purposes of life, radioactivity 
has raised an issue which it is safe to say will mark 
an epoch in the progress of thought. With all our 
mastery over the powers of Nature we have adhered 


to the view that the struggle for existence is a 
permanent and necessary condition of hfe. To-day it 
appears as though it may well be but a passing phase, 
to be altogether abolished in the future as it has to some 
extent been mitigated in the past by the unceasing, 
and as it now appears, unlimited ascent of man to 
knowledge, and through knowledge to physical power 
and dominion over Nature. 

The Discovery of Radioactivity, 

The first discovery of the property we now call 
radioactivity was made in the year 1896 by M. Henri 
Becquerel in Paris, and, like many other great dis- 
coveries, the actual experiment itself owed something 
to luck or chance or accident. Looking backward, 
however, it appears rather that only the particular 
day or month of the discovery was a matter of chance. 
The time was just ripe for the event, and it is certain 
that its coming could not long have been delayed. 
Some slight historical sketch of the conditions preceding 
and immediately following the discovery is necessary 
before considering wherein lies its great significance. 
The memorable discovery of the X-rays by Professor 
Rontgen, in 1895, which is known to all, familiarised 
scientific workers with a type of radiation able to 
traverse objects opaque to light. The X-rays are 
themselves invisible to the unaided eye, but are able 
to affect the photographic plate. This led to experi- 
ments being made in order to see if similar types of 
rays were not produced in other ways. As you all 
know, certain substances exposed to sunlight shine 
afterwards in the dark, and this property, which finds 
an application in the manufacture of luminous paint, 
is known as phosphorescence or fluorescence. Is 
phosphorescent light entirely stopped by opaque 
objects ? Or does it in part consist of invisible pene- 
trating j-ays like the X-rays ? M. Becquerel wrapped 
a photographic plate in black paper and placed on it a 


Fig. I. — Becquerel's Uranium Picture. 

Fig. 2. — Welsbach Mantle imprinted by its Own Rays. 

To face p. 7 


phosphorescent substance which was then exposed 
to sunhght. By great good fortune M. Becquerel 
chose as the particular phosphorescent body a prep- 
aration of uranium, and found as the result of the 
experiment that the photographic plate beneath the 
preparation was darkened. The preparation had given 
out rays which, unlike sunlight, were capable of pene- 
trating the black paper. It was soon found that these 
rays, like the X-rays, even penetrated thin plates 
of metal, for when such a thin plate was interposed 
between the preparation and the film darkening still 
occurred. But one day, the sun being obscured, the 
plate and the phosphorescent uranium preparation 
upon it were set aside in a dark drawer for some weeks, 
and M. Becquerel, wishing to see if any darkening had 
occurred without the sunlight, developed the plate as 
it was. It was found that darkening had gone on 
just as much in the darkness as in the light. Further 
experiments soon established that neither sunlight 
nor phosphorescence had anything to do with the 
experiment. The action is an entirely new inherent 
property of the element uranium. No other phosphores- 
cent body would have darkened the plate even in the 
sunlight, while all preparations containing uranium 
do so, whether they are phosphorescent or not, in 
total darkness as well as in the light. Fig. 1 shows 
one of the photographs by uranium rays obtained by 
M. Becquerel. Between the patch of the uranium 
preparation and the plate was placed an aluminium 
medallion, stamped with a head of a figure in relief, 
which partially shielded the plate beneath from the rays. 
The impression under the thinner portions of the medal- 
lion is darker than under the thicker portions, thus 
causing the head of the figure to be clearly apparent 
in the photograph. 


The Four Experimental Effects of Radio- 

Although the radioactive process is itself without 
analogy in science, the main effects which it pro- 
duces can almost all be more or less nearly imitated, 
and were all more or less perfectly studied prior to 
its discovery. The main effects of radioactivity 
with which we are most concerned are four. Firstly 
then, radioactive substances affect a photographic 
plate in the same manner as light and many other 
agencies. Secondly, they excite phosphorescence or 
fluorescence in certain substances when brought in their 
neighbourhood. Thirdly, radioactive bodies cause the 
air and other gases to lose the insulating power they 
normally possess and to become partial conductors of 
electricity. In consequence, any electrified object 
has its electricity rapidly discharged in the neigh- 
bourhood of a radioactive substance. The passage 
of the rays through the gas shatters the electrically 
neutral gas molecules into oppositely charged particles 
or, as it is termed, ionises the gas. But the same 
effect is produced by X-rays, by incandescent bodies, 
and even by a lighted match. The instrument em- 
ployed to detect this effect is the gold-leaf electro- 
scope, the first and simplest electrical instrument to 
be invented, and for this purpose capable of so great 
refinement that it affords the most delicate and sensi- 
tive test it is possible to employ in the detection of 
radioactivity. Lastly, radioactive bodies generate heat, 
as does coal or any other substance burning. The 
photographic action and the discharge of electricity 
from insulated charged bodies are clearly shown by 
radioactive substances even in the form in which they 
occur in Nature, as all unsuspected they have been 
handled and examined by men for centuries. Hence 
you will understand how it is that the discovery of 
radioactivity could not under any circumstances have 


been indefinitely delayed. But only the more power- 
fully radioactive substances, like radium, give appreci- 
able phosphorescence or heat effects. In the naturally 
occurring radioactive substances these effects are far 
too small to be readily detectable. 

The Rays of Radioactive Substances. 

Exact physical experiments have demonstrated that 
all these effects of radioactivity owe their origin to the 
fact that the radioactive substances emit " rays." These 
rays are invisible to the unaided eye it is true. In this 
they resemble Rontgen's X-rays. There are three 
different types of rays given out by the radioactive 
substances, which are known respectively as the a-, /3-, 
and 7-rays. Each will require detailed future considera- 
tion. But they all bear less resemblance to light than 
to the recently discovered types of rays, of which the 
X-rays of Rontgen are typical, produced when an 
electric current is forced by powerful appliances to 
traverse a nearly vacuous space, a path which it much 
prefers not to take if it can avoid it. 

The first effects of most new things are old. Motor- 
cars and railways do the old work of horses. In commer- 
cial life a really new effect is generally valueless until it 
has ceased to be new, as many inventors know to their 
cost. In scientific discovery a new effect does not 
usually proclaim itself from the housetops. It often 
needs new instruments and the way must first be paved 
for its discovery, while old effects are generally recog- 
nised first. It is natural that the first effects of radio- 
activity to be discovered should be those more or less 
familiar. But for the development to perfection of 
that marvellous thing, the photographic plate, radio- 
activity would not have been discovered in the way it 
was, and we should still be without one of the readiest 
methods of detecting it. But for the work on the con- 
duction of electricity through gases immediately follow- 
ing the discovery of the X-rays, the only other method 


of detecting radioactivity in the natural state would be 
unknown, and therefore also in all probability radio- 
activity itself. On the other hand, if radioactive 
substances exhibit any entirely new kind of properties — 
and it is quite possible that they do — it is very likely 
that their very novelty would delay their discovery. 

The Continuous Emission of Energy. 

Why then, you may ask, if all of the effects of radio- 
activity are shown in other ways do I insist that radio- 
activity is a phenomenon unparalleled in science ? The 
distinctive feature of radioactivity is not, however, so 
much in the rays the radioactive substances emit, 
though we shall find upon a closer examination that 
these are distinctive and most remarkable. The main 
interest of the new property consists in the spontaneous 
and continuous emission of energy of which the rays are 
but one manifestation. Heat and light may be obtained 
in numerous ways, but it is a new thing to find it being 
given out by a substance, as it is by radium, year in, 
year out, without apparent intermission or diminution, 
and without the substance being in any apparent 
way consumed or altered. This was the arresting fact. 
The radioactive substances apparently were perform- 
ing the scientifically impossible feat of evolving a 
store of energy presumably out of nothing. So long 
as radioactivity was known only on the scale and 
in the degree exhibited by uranium, it was perhaps 
possible to explain away this aspect of the question 
because of the minuteness of the amount of energy 
involved and the difficulty of proving that it was not 
in some way derived from the surroundings. But the 
work of M. and Mme. Curie, by their discovery of radium, 
made the world familiar with an element over a million 
times as radioactive as uranium. In this case the 
energy evolved is great enough to produce effects 
which are obvious to all and which cannot be ex- 
plained away. In a strictly scientific sense there is no 


difference of principle between the radioactivity of 
radium and that of uranium. The difference is one of 
degree only, but it is so great that radium, though, as 
we shall come to see, not so wonderful in reality as 
uranium, rapidly acquired a monopoly of public interest 
and attention. 



Radioactivity, an Unalterable Atomic 

It is worth while to stop to consider the starting-point 
of Mme. Curie's discovery. Chemistry analyses all 
known substances into their component constituents 
or elements, all of which are fundamentally different, 
the one from the other, and inconvertible the one into 
the other. Uranium is such an element, gold, silver 
lead, and many of the common metals are others, but 
uranium is distinguished by having relatively the 
heaviest of all known atoms. The atom is the minimum 
unit quantity of an element. The relative atomic 
weight of an element is one of its most important charac- 
teristics, and as a first approximation the atom of hydro- 
gen is chosen as the standard and is assigned unit value. 
For exact work it is more convenient to choose oxygen 
as the standard, with the value 16. On this basis the 
atomic weight of hydrogen becomes 1-008, and that of 
uranium 238-18. 

Now radioactivity is an intrinsic property of the 
element uranium, and therefore of the atom of uranium. 
This Mme. Curie first recognised, and it formed the 
starting-point of her work. In the case of uranium, 
the element itself and all its various compounds are 
radioactive, and the radioactivity of each compound is 
conditioned simply by the relative amount of uranium 
it contains. It does not matter where the uranium 
comes from — it is always to the same degree radio- 
active. Non-radioactive uranium is unknown. Not 



only so, but it is absolutely impossible really to affect 
the radioactivity of uranium or any other of the radio- 
active elements in the slightest degree. In this the 
process is utterly unlike any other process previously 
known in Nature. Radioactivity is part and parcel of 
the very nature of the element which possesses the pro- 
perty, and therefore of the atom or unit quantity of the 
element. The attempts that have been made artificially 
to alter or to stop the radioactivity of an element have 
met with signal failure. This is still an impossible feat 
• — a thing modern science cannot do — and yet, as we 
shall come to see quite clearly in the sequel, a thing 
which science must do if mankind is to realise to the 
full the destiny these discoveries have for the first time 
unveiled. There is another still impossible feat, to the 
accomplishment of which all the appliances of modern 
science have been directed in vain, as well as all the 
utmost power of man from the earliest time. It is 
transmutation, or the conversion of one element into 

Radioactivity is the one process going on in matter 
we cannot influence or stop, while transmutation is the 
one process in matter we have so far signally failed to 
effect. The juxtaposition of radioactivity and trans- 
mutation is not a fanciful one, because it will appear, 
as we proceed, that the two processes are most inti- 
mately connected. 

The Radioactivity of Thorium. 

Radioactivity being a property of the element 
uranium, it was natural to ask whether uranium alone 
of all the eighty elements known possessed it. This was 
the starting-point of Mme. Curie's illustrious researches 
in the subject. She found only one other element 
among those known which possessed the property — the 
element thorium, which, at one time rare and little 
known, has come into mdustrial prominence of recent 
years in the manufacture of the Welsbach incandescent 


gas mantle,^ of which it forms the main constituent. 
To the electrical or ionisation test — the power of dis- 
charging a gold-leaf electroscope — thorium prepara- 
tions are of about the same degree of radioactivity as 
uranium; but to the photographic plate thorium is far 
less active than uranium, owing to the fact that the type 
of rays which affect the photographic plate most strongly 
are not those with most effect on the electroscope. 
The radioactivity of thorium is a fact which can be 
beautifully demonstrated by any one acquainted with 
the process of photography. An incandescent mantle, 
after burning off the fibre, is cut open and pressed as flat 
as possible on a card. A photographic plate, which has 
first been wrapped in a light-tight envelope, is laid upon 
the flat mantle, and the whole is left undisturbed for a 
fortnight or longer. On developing the plate it will be 
found that an image of the mantle has been formed on the 
plate in the dark by the rays from the thorium contained 
in the mantle. Any one can do this simple experiment 
for himself. 

Fig. 2 (facing p. 7) shows the result I obtained with 
a very thin piece of aluminium foil between the film and 
the mantle. The foil, while quite opaque, allows the 
a- as well as the /S-rays to go through. Paper would 
stop the a-rays entirely. 

The radioactivity of thorium, though producing the 
same general effects as that of uranium, differs from 
it entirely in detail. Indeed, by a few simple tests 
on the radioactivity, any one of the radioactive 
elements can be recognised and distinguished far 
more quickly and certainly than by any of the other 
chemical or spectroscopic tests, even when present 
in very minute quantities. In the historical develop- 
ment of the views now held in radioactivity thorium 
played a leading part. But, as it is quite foreign to 
my intention to give anything approaching a detailed 
systematic account of the subject, and as radium lends 

1 The cause of the action of the gas-mantle in generating light is 
quite unconnected with the property of radioactivity. 

Fig. 3. — Sir William Crookes' Pictures of Pitchblende 

The lower figure is a daylight photograph. 

The upper was imprinted in the dark by the rays from the substance. 

To face p. 15 


itself more readily to experimental demonstrations, I 
shall confine myself at first to the properties of the 
latter substance. 


Although uranium and thorium were the only two 
known elements possessing radioactivity, Mme. Curie 
found that the natural minerals containing uranium 
are more radioactive than can be accounted for by the 
uranium present. Certain minerals, called pitchblende, 
particularly the variety from the celebrated Joachims- 
thal mine in Austria, contain often more than 50 per 
cent, of uranium in the form of uranium oxide. The 
radioactivity of pitchblende to the photographic plate 
is beautifully shown by two photographs of Sir W. 
Crookes (Fig. 3). The lower figure shows the polished 
face of a piece of pitchblende photographed in the 
ordinary way by daylight. The upper figure was 
taken by placing the polished face of the mineral on a 
photographic film wrapped in light-tight paper. The 
lighter portions of the figure indicate where the plate has 
been acted on by the rays from the radioactive matter in 
the pitchblende. Some pitchblendes are from three to 
four times as radioactive as pure uranium oxide. This 
could only be the case, Mme. Curie correctly argued, if 
there existed in the minerals one or more unknown 
elements more powerfully radioactive than uranium. 
By the ordinary process of chemical analysis it is easy to 
separate out the various constituent elements in pitch- 
blende. There are a great number of elements in pitch- 
blende, though most of them are present in very small 
amount. A fact that will be found significant later is 
that lead is always present in important quantity. 
Mme. Curie found that of the elements so separated two 
in particular, the bismuth and the barium, were strongly 
radioactive. Now ordinary bismuth and barium are 
not at all radioactive, and the radioactivity of these 
elements, when separated from pitchblende, is really 
due to the presence of two new elements in minute 


amount mixed with them. The one associated with 
bismuth was discovered first by Mme. Curie and 
named Polonium, after her native country. Its con- 
sideration is more profitably delayed till later. The 
other, which was discovered very soon afterwards, is 
associated with the barium, and is Radium. 

Quantity of Radium in Pitchblende. 

The exact quantity of radium in pitchblende and 
other uranium minerals is a fact of considerable impor- 
tance. There is one part of the element radium for 
every three million two hundred thousand parts of the 
element uranium in pitchblende. The pitchblende may 
be of any degree of richness, from only a few per cent, 
to over 50 per cent, of uranium. But of even the 
richest pitchblendes between 100 and 200 tons would 
be needed to produce an ounce of pure radium. The 
compound usually sold, hydrated radium bromide, the 
formula of which is written RaBr2-2H20, contains, if 
pure, 54-33 per cent, of radium. But what it lacks in 
quantity radium makes up for in quality — that is to say, 
in radioactivity. It is like the myriad of roses we are 
told go to make a single drop of the real attar, which 
is almost priceless. The radium that is extracted is a 
million times more radioactive than the mineral, and 
several million times more than pure uranium itself. 
Conversely, just as you can buy quite a large bottle of 
rose-water for a small sum, so quantity is not the only 
consideration to be taken into account in the buying of 
radium preparations. A very small quantity of radium 
is sufficient to confer on a large quantity of an inactive 
salt many of its own peculiar properties. Particularly 
is this the case with the property of glowing visibly in 
the dark. Weak radium preparations, which contain 
usually barium, shine by themselves in the dark more 
strongly even than the pure radium salts, owing to a 
phosphorescent action of the barium salts, although 
they may hardly contain enough radium to affect an 
X-ray screen through a piece of metal. If you mix n 


very minute quantity of radium with a quantity of a 
very highly phosphorescent body, Hke sulphide of zinc,^ 
it will shine in the dark so brilliantly that an in- 
experienced person might well be deceived into believing 
that it must contain a large quantity of radium. So 
great has become the need that radium preparations 
should be of definitely ascertainable quality that in 
1910 an International Radium Standards Committee 
was formed, with the result that there is now preserved 
in Paris an International Radium Standard prepared 
by Mme. Curie, and consisting of a tube containing 
twenty-two milligrams of the most carefully purified 
radium chloride. By comparison with this standard 
secondary standards have been prepared and supplied 
to the official testing institutions of the various countries, 
and now there is as much definiteness about the milli- 
gram of radium as there is about a pound of tea. 

The Smallest Quantity of Radium Detectable. 

It is an interesting digression to consider here the 
smallest absolute quantity of radium which can be de- 
tected and identified with certainty in the laboratory. 
One fifty-millionth of a milligram, or one three- thousand- 
millionth of a grain of radium is quite easy to recognise, 
whilst with special care one- tenth of this amount could 
probably be detected. Thi§ is far less than could be 
detected in the case of any non-radioactive element by 
any method known, not excluding even the spectro- 
scope. If the half of a grain of pure radium bromide, 
which is in this room to-night, were divided equally 
among every human being at present alive in the world, 
and one such portion were returned to us, it would 
prove sufficient for detection and identification by means 
of a gold-leaf electroscope with the greatest ease. With 
half a grain of a pure radium compound the main effects 
of radioactivity, which in the case of uranium or thorium 
would either be too feeble to show or would require the 

1 This mixture now finds extended application in " Radium Watche.s' 
and the like, for painting the dial figures and tips of the hands. 


use of inconveniently delicate instruments, can be shown 
in a striking and convincing manner to you all in the 
simplest possible way. 

Experiments with Radium. 

Of the small amount of radium bromide, which by a 
labour of love certain chemists have succeeded in ex- 
tracting from pitchblende, I am fortunate to possess 
about a grain, or sixty-five milligrams. Half of this 
quantity, which I shall use for most of my lecture 
experiments, is contained in a small ebonite capsule. 
The other half is dissolved in water and not brought 
into this lecture-room, but kept in the laboratory half 
a mile away. With the room dark the radium in the 
capsule is hardly visible to you, because the rays do not 
of themselves affect the unaided eye, but if I bring some 
crystals of the fluorescent substance, barium platino- 
cyanide, near to it, you will see that the crystals shine 
out at once with a beautiful green light. An ordinary 
X-rays fluorescent screen, which is simply a piece of card 
painted over with the same fluorescent substance in the 
form of powder, is very convenient for these experiments. 
When thin pieces of metal foil are placed between 
the radium and the crystals you see their brightness is 
only slightly reduced, while several shillings can be in- 
terposed one above the other without altogether stop- 
ping the rays from the radium. Those in the front will 
see the crystals still shining faintly, although the rays 
from the radium have first to traverse more than half 
an inch of solid silver before reaching the crystals. The 
electrical effect of radioactivity can be shown in a very 
rough and simple way with this, comparatively speaking, 
large quantity of radium. A silk tassel is stroked with a 
rubber tobacco-pouch and so electrified. All the threads 
then repel one another and stand out as you see (Fig. 4). 
The moment the radium is brought near the threads 
collapse at once (Fig. 5). Lastly, the photographic action 
of the rays is seeh in the photograph (Fig. 6, facing p. 81) 





















To face p. i8 


which was obtained by slowly writing, with a small 
tube containing a small fraction of a grain of radium 
bromide as if it were a pencil, over a photographic 
plate wrapped in black paper, and then developing the 
plate without exposure to light. 

By the aid of delicate thermometers it could also be 
shown that this small quantity of radium is always a 
few degrees hotter than the surrounding air. 

Cost of Radium. 

The one fact about radium, which every one is aware 
of, is its tremendous cost. When you consider that even 
of the best ore several hundredweights must be worked 
up to obtain the small quantity here exhibited, you 
can understand that the price is necessarily very higii. 
The price rose rapidly from about 8s. the milligram for 
radium bromide in 1903 to about £15 the milligram in 
1912, and even at the latter price very inferior prepara- 
tions have found a ready sale. During the war, in 
which radium found several applications for illuminating 
rifle sights, compass cards and the like, over £20 a 
milligram was paid, and it is likely to remain at this high 
level. We shall see, as we proceed, that from its very 
nature any strongly radioactive body like radium must 
always be excessively rare. Indeed, in the degree of 
radioactivity we have a scientific standard of rarity, 
and therefore of " value." There are unfortunately 
some fields of scientific investigation, of which radio- 
activity is one, which cannot be thoroughly explored 
without continuous and considerable expenditure. The 
old boast of science, that some of her grandest dis- 
coveries were made with very simple apparatus, largely 
built up of wire and sealing-wax, costing little or 
nothing, does not apply to any of the discoveries with 
which we are now concerned. The investigations of 
Mme. Curie naturally have cost many thousands of 
pounds, provided in part by the Austrian Govern- 
ment and the Rothschilds. This radium we are using 



to-night we owe to the work of a German chemist, 
Dr. Giesel, who undertook its extraction on a large 
scale in the early days when the raw material was to be 
obtained in the market, and who very unselfishly dis- 
tributed much of the radium he prepared among workers 
in all parts of the world. 

The chief source of radium at the present time (1920) 
is American carnotite, which contains 2 per cent, of 
uranium in the form of a uranium potassium vanadate, 
mixed with sandstone. Its use in medicine and, later, 
in war has produced results as startling in the field of 
common sense as in that of physical science. " A 
great industry has sprung up." The creator of the 
wealth, the scientific investigator who discovered the 
material and the methods of winning it, and made a free 
gift of all his hard-won knowledge to the community, is 
now unable to afford to buy radioactive materials, even 
in the modest quantities he needs for scientific investi- 

The Doctrine of Energy. 

To-night it is not my intention to take you through 
the various phases of the new properties of radium. 
We have to face squarely the great general question 
which its simple existence has demanded of physical 
science. Last century will remain for ever memorable 
on account of the development and establishment of 
the great doctrine of energy. Those were splendid days 
for physical science in Scotland, for that doctrine, which 
lies at the root of all modern industry and enterprise, 
took its rise largely in Scotland, and was developed by 
Tait, of Edinburgh, and Lord Kelvin, of Glasgow. 

For a full account of these stirring developments you 
should read Tait's Recent Advances in Physical Science, 
which, in spite of the fact that it is now over forty years 
old, still continues fresh and inspiring. The first law, 
that of the conservation of energy, states that energy is 
a real entity, and has a real existence no less than matter, 
and no more than matter can energy be created or de- 


stroyed, although the forms it may assume are legion. 
The second law, that of the availability of energy, is 
sufficiently accurately stated for present purposes by 
saying that the same energy is available for useful work 
but once. To obtain useful work from any source of 
stored-up or potential energy, it is necessary to trans- 
form it into new forms which are kinetic, and by which 
something is made to move. As motion is invariably 
attended by friction or similar processes, ultimately the 
energy passes into heat. It is said to be degraded into 
low-grade or waste energy, for although all forms of 
energy tend, after assuming the kinetic form, to turn 
into heat, the transformation of the waste heat so pro- 
duced back into useful forms cannot be practically 
effected. The conversion is not altogether impossible, 
but requires for its accomplishment the degradation of 
more fresh energy than is gained, and so is practically 
out of the question. 

The practical aspect of the question may be summed 
up by saying that if you want useful energy, you must 
pay for it like any other commodity, and the value of the 
energy, though not the energy itself, is destroyed by use. 
The up-to-date street car driven by the electric motor, 
which has displaced the old horse-tram, although it has 
not the same obvious incentive to locomotion as its pre- 
decessor, nevertheless does not go by itself. It requires 
energy or power, which is bought and sold and has a 
value as strictly as the oats and hay which energised the 
now emancipated horse. The driving power of the 
machinery of the modem world is often mysterious, but 
the laws of energy state that nothing goes by itself, and 
our experience, in spite of all the perpetual motion 
machines which inventors have claimed to have con- 
structed, bore this doctrine out, until we came face to 
face with radium. Nothing goes by itself in Nature, 
except apparently radium and the radioactive sub- 
stances. That is why, in radioactivity, science has 
broken fundamentally new ground. 

I cannot too plainly insist that available energy, 



though immaterial and intangible, has a definite and 
real physical existence. Were it not so, coal would not 
be the very expensive commodity it unfortunately is 
rapidly becoming. No one burns coal for the sake of 
polluting the atmosphere, but simply and solely because 
it gives out during combustion a certain amount of 
energy as light or heat. Last century civilisation may 
be said to have attained its majority and to have entered 
upon the control of an inheritance of energy stored up 
by the sun in fuel during the long ages of the past, and 
now it is dissipating that inheritance as quickly as it can. 
With the light -heartedness and irresponsibility of youth, 
it is taking no thought of the future, but confidently 
assumes that the supply of natural energy, upon which 
at every turn it is now entirely dependent, will continue 
indefinitely. Well ! if it does not do so, new stores of 
energy cannot be created to order, and there will be an 
end to the age of energy in which we are living, and to 
civilisation as we have come to understand it. 

Measurement of the Energy emitted by 

Energy is susceptible of exact measurement and, 
though it exists in many varieties, all forms of energy 
can be most readily and completely converted into heat 
and measured as such. The energy given out by radium, 
although it is in nature new, is no exception to this rule. 
Practically the whole of the energy is transformed into 
heat when the radium is kept in a leaden vessel, so that 
the rays are absorbed in the surrounding metal. The 
actual amount of heat given out, for instance, by this 
small quantity on the table is, of course, very small, but, 
in comparison with the quantity of substance producing 
it, it is very great indeed. Exact experiments have 
proved that 1 gram (=15-4 grains) of radium gives out 
133 calories per hour.^ The amount of heat evolved by 

1 The calorie is the quantity of heat required to raise 1 gram of 
water 1° Centigrade. Spelt with a capital C, the Calorie is 1,000 calories 


any quantity of radium in three-quarters of an hour is as 
much as is required to raise a quantity of water equal 
in weight to the radium from the freezing-point to the 
boihng-point. Radium bromide, if it is dry, consists 
roughly of three-fifths by weight of the element radium 
and two-fifths of the element bromine. Half a grain of 
radium bromide thus evolves two and a half calories 
every hour. This specimen of half a grain of radium 
bromide has been in my possession for sixteen years, 
and the outpouring of energy has been going on cease- 
lessly day and night at a steady rate. A simple calcula- 
tion shows that in this time about 350,000 calories have 
been evolved. To obtain an idea of what this means 
consider the amount of energy given out in the burning 
of coal. A weight of coal equal to the weight of this 
radium bromide would give out during complete com- 
bustion only about 250 calories, so that this radium has 
evolved in sixteen years 1,400 times the energy obtain- 
able from the same weight of coal. I have chosen coal 
for the comparison because the combustion of carbon 
furnishes the modern world with its main supply of 
energy. During the last sixteen years this radium 
has given fourteen hundred times as much energy as 
could be obtained from an equal weight of any other 
kind of substance in any way known. Coal is no longer 
coal when it is burnt and consumed. Gunpowder and 
dynamite, once they have exploded and evolved their 
stored-up energy, disappear as such, and there remain 
incombustible and non-explosive solids and gases, out 
of which no more energy can be drawn. But this radium 
is as active as ever. So far, careful measurements have 
failed to detect the least diminution in the radio- 
activity of radium with time. Rather it increases 
steadily, rapidly in the first month and slowly for the first 
few years after preparation, for certain profound reasons 
we shall have to go into subsequently. These show also 
that after some thousands of years the evolution of 
energy must cease. But the calculated diminution is 
only some four per cent, per century. 


The Source of Cosmical Energy. 

In the face of a new fact of this character it is obvious 
that this doctrine of energy, which we thought so well 
founded, requires further consideration. Based as it 
has always been on the results of our experience and the 
practical impossibility of achieving perpetual motion of 
any kind, it is confronted with a natural example, 
going on apparently for an unlimited space of time 
under our very eyes, which not only does not come to 
a stop, but which cannot be stopped by any means what- 
ever. Now, although the doctrine of energy accords 
well enough with our terrestrial experiences, the student 
of the physical sciences has only to turn his thoughts 
from the laboratory to the heavens to see there, in the 
larger laboratory of Nature, an example of practical 
perpetual motion on the grandest and most majestic 
scale. What, for example, is the source of the apparently 
inexhaustible supply of energy from the sun, upon the 
receipt of a minute and insignificant fraction of which 
life on this planet absolutely depends for its continued 
existence from year to year ? This is a question which 
has been frequently asked and only imperfectly answered 
by physical science. It has been the custom vaguely to 
connect the apparently endless and inexhaustible out- 
pourings of energy going on everywhere in the universe 
with its vast scale and dimensions. In the background 
there has always been the tacit assumption that the 
supply of fresh energy is only apparently inexhaustible, 
and that in some remote future a time will at length 
arrive when the supplies of fresh energy are exhausted 
and all things will come to a stop and remain at rest for 
ever. We have applied the teachings of the labora- 
tory, our knowledge of the laws of energy and its con- 
servation, and the impossibility of perpetual motion, 
without modification to the cosmos, only making allow- 
ance for its enormous scale. 

Astronomers, in consequence of the new discoveries, 


are no longer compelled to regard cosmical evolution as 
proceeding on these old conventional lines. It is not so 
certain as it was that it is only a question of time before 
the sun and planets cool down to a dead uniform tem- 
perature. In former days this point of view was the 
only possible one. A hot body radiating heat and light 
into space, even when all possible sources of energy, such 
as the accretion of meteorites, shrinkage, etc., have been 
allowed for, must ultimately radiate away its energy. 
The same is still true but with a difference. Thus 
Professor Tait, in his Recent Advances in Physical 
Science (1876), says (p, 169): " If we were to trace the 
state of affairs back, instead of to ten millions, to a 
hundred millions of years, we should find that (if the 
earth then existed at all) if that collocation of matter 
which we call the earth was then actually formed, and 
if the physical laws which at present hold have been 
in operation during that hundred million years, then 
the surface of the earth would undoubtedly have been 
liquid and at a high white heat, so that it would have 
been utterly incompatible with the existence of life of 
any kind such as we can conceive from what we are 
acquainted with. Thus we can say at once to geolo- 
gists, that granting this premiss — that physical laws 
have remained as they now are, and that we know of all 
the physical laws which have been operating during that 
time — we cannot give more scope for their speculations 
than about ten or (say at most) fifteen millions of years. 

" But I dare say many of you are acquainted with the 
speculations of Lyell and others, especially of Darwin, 
who tell us that even for a comparatively brief portion 
of recent geological history three hundred millions of 
years will not suffice. 

" We say, so much the worse for geology as at present 
understood by its chief authorities, for, as you will 
presently see, physical considerations from various inde- 
pendent points of view render it utterly impossible that 
more than ten or fifteen millions of years can be granted." 

Again (p. 154): "Take (in mass equal to the sun's 

26 - RADIUM 

mass) the most energetic chemicals known to us, and in 
proper proportion for giving the greatest amount of 
heat by actual chemical combination, and, so far as we 
yet know their properties, we cannot see the means of 
supplying the sun's present waste for even 5,000 years. 
. . . This question is totally unanswerable, unless there 
be chemical agencies at work in the sun of a far more 
powerful order than anything that we meet with on the 
earth's surface." 

Radium and the " Physically Impossible." 

I do not quote these utterances with any wish to 
revive the old controversy between geologists and 
physicists, long since tacitly abandoned by both sides 
mutually as barren and unprofitable, but because of their 
present extraordinary aptness. To-day, science has 
come to know, by means of radioactivity, of agencies at 
work on the earth's surface of a far more powerful order 
than anything that was known in the time of Professor 
Tait. The discovery of radioactivity and the revela- 
tion it has given of unsuspected stores of energy in 
Nature available for cosmical purposes, of necessity put 
the whole question of the evolution, the past history and 
the future destiny of the universe in a new light. This is 
one of the conclusions of clearly general interest which 
follow from the recent discoveries. 

There is nothing of the vast scale and dimensions of 
the universe about this tiny scrap of radium. Yet it is 
giving out energy at a rate, relative to its mass, which no 
sun or star is doing. Suppose, for example, our sun, 
instead of being composed of the materials it is, which 
we know by the spectroscope are practically the same 
as those of the earth, were made of pure radium. Pro- 
vided only that every part of its mass gave out energy 
at the rate this radium on the table is doing, there would 
then be no difficulty in accounting for its outpourings of 
energy. Rather, the light and heat that would be given 
out from such a sun would be of the order of a million 
times greater than they actually are. On another count 


also one's thoughts almost unconsciously revert from 
radium to the transcendental phenomena of the larger 
universe, for in no other phenomena are we so reduced 
to the position of onlookers, powerless alike to influence 
or control. All the powerful resources of the modern 
laboratory — extremes of heat and cold, and of pressure, 
violent chemical reagents, the action of powerful ex- 
plosives and the most intense electrical agencies — do not 
affect the radioactivity of radium or the rate at which 
it works in the slightest degree. It draws its supplies 
of energy from an hitherto unknown source and obeys 
as yet undiscovered laws. There is something sublime 
about its aloofness from and its indifference to its exter- 
nal environment. It seems to claim lineage with the 
worlds beyond us, fed with the same inexhaustible fires, 
urged by the same uncontrollable mechanism which keeps 
the great suns alight in the heavens over endless periods 
of time. This tiny speck of matter we can hold in 
our hands exhibits in perfect miniature many ancient 
mysteries, forgotten almost in their familiarity, or mis- 
takenly and too easily dismissed as belonging and 
appropriate to the infinitely great dimensions of the 
universe. The " physical impossibility " of one era 
becomes the commonplace of the next, and in the con- 
troversy between the geologists and the physicists we 
have a good illustration that no theory can claim a 
universal application. It is of necessity partial, and 
bounded on all sides by the unknown and unexplored. 
It is rarely proved false, so surely and truly are the 
foundations of modern science laid, but it is liable 
at any moment to be restricted in its application to 
the particular cases for which it was formulated and 
found not to apply in new spheres at the time of its incep- 
tion unsuspected. As we shall see, the law of the con- 
servation of energy is not necessarily controverted by 
any of the new facts with reference to radium, but prior 
to these discoveries our knowledge of the available 
sources of energy in Nature has been partial and super- 
ficial to a degree. 



The Radiations of the Radio-Elements. 

In the previous lectures we have considered the bare 
fact that radium and the radioactive substances are 
continually evolving from themselves a perennial 
supply of energy, and the fundamentally new ground 
which this discovery opens up in physical science. 
To-night our inquiries will be directed to one special 
portion of the subject, namely, the nature of the rays 
emitted by the radioactive elements, by means of which, 
or rather of the effects of which, the property was first 
discovered. These rays themselves, apart from their 
effects, we have hitherto scarcely considered, but they 
play an essential part in the theoretical scheme by which 
the activity of the radio-elements is now interpreted. 

The tracing back of the main effects of radioactivity, 
photographic, fluorescent, electrical, and thermal, to 
definite radiations emitted by the radio-elements came 
very early in the subject, but it must not be for- 
gotten that such tracing back is of the essence of 
the discovery. Too frequently it is wrongly assumed 
without such evidence that any substance capable of 
simulating one or other of the various effects of radio- 
activity is therefore a radioactive substance. Natur- 
ally, the exact study of the new radiations has been 
mainly the work of physicists. They have succeeded, 
not only in clearly analysing into distinct classes the com- 
plex radiations involved and distinguishing the part 
played by each alone, but also they have advanced 
very far towards a solution of the real nature of each 


a-, y8- AND r-RAYS 29 

class of radiation emitted. Much of this latter work, 
however, is based upon reasoning of too specialised and 
intricate character for general presentation, and as these 
lectures are intended primarily for the general public, and 
not for trained physicists, I propose concentrating 
attention for the most part on the conclusions which 
are universally accepted and of the greatest general 
interest. Although the reasoning is difficult, the chief 
conclusions are very simple and easily followed, and 
they fit in with the general scheme of the cause and 
nature of radioactivity in a way which makes the whole 
subject clearer and more easily visualised. 

a-, /3- AND 7-Rays. 

The first analysis of the complex radiations emitted 
by each of the radio-elements — uranium, thorium, and 
radium — was done by Sir Ernest Rutherford, and much 
of the work we are considering is his, and has called 
forth in their highest degree his well-known experi- 
mental genius and energy. He classed the rays 
into three main types, the a-, ^- and 7-, distinguished 
from one another by enormous differences in their power 
of penetrating matter. I may say at once that the a- 
rays of radium, for instance, are readily distinguishable 
in penetrating power from the «-rays of uranium, and 
the latter again from those of thorium. Moreover, 
the a-rays of radium are themselves complex and con- 
sist of no less than four separate types readily dis- 
tinguished. The same is true of the yS- and 7-rays of 
radium, which are themselves complex and recognisably 
different from the /3- and 7-rays of uranium or thorium. 
But the differences between the a-rays as a class, for ex- 
ample, are small and unimportant relatively compared to 
the enormous difference between any a-ray and any 
/S-ray or 7-ray. The most penetrating a-ray known is 
not much more than twice as penetrating as the least 
penetrating known, whereas the /3-rays as a class may 
be considered to be approximately a hundred times more 



penetrating than the a-, and the 7-rays a hundred times 
more penetrating than the /S. Again, the kind of 
matter penetrated, although it has a certain influence 
which may be different for different types of rays, is 
only of secondary importance. For these rays, like 
the new X-rays, and unlike light, are absorbed by matter 
roughly in proportion to its density, and quite indepen- 
dently of its optical qualities of transparency and opacity. 
The first result of these researches was to bring into 
prominence the o;-class of rays, which at first sight are of 
apparently little importance, and to diminish relatively 
the importance of the /3-class of rays which had been 
operative in the photographic effects hitherto mainly 
studied. For the a-rays are completely absorbed by 
very thin screens — even by a sheet of thin paper, or by 
three inches of ordinary gaseous air, — and they produce 
but little action on the photographic plate in com- 
parison with the ^-rays, which are able to pass through a 
visiting card or piece of tinfoil with ease. To the electri- 
cal test — the discharge, for example, of an electrified 
silk tassel or electroscope — ^the a-rays are immensely 
more effective than the /3- and 7-rays together, and from 
this fact Rutherford concluded, and the conclusion has 
been wholly borne out by subsequent developments, 
that the energy possessed by these feebly penetrating, 
and not at first sight very striking, a-rays is always 
immensely greater than that of the other two types 
taken together. In fact, the yS- and 7-rays at most 
possess but a few per cent, of the total energy of radia- 
tion, and therefore are in this fundamental respect rela- 
tively of less consequence than the previously neglected 
a-class. Although less suited to lecture experiments 
than the other more penetrating types, the a-class have 
proved far the most instructive and important in the 
theory of radioactive change. 

Fig. 6. — Written by Radium in the Dark. 

(From a Radiograph by R. Hill Crombie, Esq., Jou7-nalofthe Rontgeii Society, Dec, 1906.) 

I'lG. 7. — Closed Bo.x: of Co.mpasses taken with the 7-Rays of Radium. 

To face p. 31 


Experiments with the Penetrating /3- and 
7 Rays. 

The small capsule in which my radium is contained 
is closed by a thin sheet of mica, which effectively stops 
all the a-rays, so that in working with the capsule only 
the yS- and 7-rays are operative. The platinocyanide 
salts fluoresce most brilliantly under the /3-rays. On 
interposing successive thicknesses of thin copper or 
aluminium foil the fluorescence is weakened, very rapidly 
at first, but a point is soon reached when the feeble 
fluorescence remaining is not much further weakened 
by additional thicknesses of foil. This is because the 
/3-rays have all been absorbed, and there remain only 
the relatively feeble but extraordinarily penetrating 
7-rays. These 7 rays are always very feeble, and com- 
paratively unimportant, but their chief interest lies in 
^he fact that they are by far the most penetrating type 
__of radiation at present known. If the capsule is com- 
pletely closed in a box of steel, half an inch thick, and a 
platinocyanide crystal laid on the top, those in front can 
readily see that the crystal still fluoresces, and stops the 
moment it is taken away from the radium. Through a 
pile of twelve shillings, or pennies, the effect can still be 
observed, while by means of a sensitive gold-leaf elec- 
troscope it has been shown that a minute proportion of 
the rays can penetrate a foot thickness of solid lead. 

The rays from radium are not well adapted for the 
taking of radiographs of the kind produced by X-rays. 
The )S-rays are hardly sufficiently penetrating for this 
purpose, so that the flesh as well as the bones of the hand, 
for example, casts a heavy shadow. The 7-rays, on the 
other hand, are far too penetrating, and *^he bones hardly 
cast a shadow at all. The picture (Fig. 7), however, 
is a good example of a radium radiograph taken by the 
7-rays of radium. A small box of compasses with the 
lid shut was placed on a table. Over it, film down, was 
placed an X-ray plate wrapped in a light-tight envelope. 



On the floor beneath, at a distance of twenty-five inches 
from the plate, was placed one-tenth of a grain of pure 
radium bromide sealed up in a tiny glass tube. The 
radium was placed between the poles of an electro- 
magnet, as recommended by Mme. Curie, to deflect 
away the /S-rays which tend to blur the distinctness of 
the picture. "IntHis way the 7-rays of radium were 
alone used. The exposure was five days. It will be 
seen that the shadow cast by the wooden box is scarcely 
noticeable, while even the metal compasses and fasten- 
ings of the box by no means entirely stop the rays. 
The metal parts appear in the negative only slightly 
darker than the unprotected portions of the plate. The 
negative was reduced and intensified before repro- 

At first the 7-rays appeared to be a secondary radiation 
produced by and accompanying the yS-rays, much as 
X-rays are produced by and accompany cathode-rays. 
The /3- and 7-rays seemed always to go together, any 
variation of the )S-rays being accompanied by a 
similar variation of the 7-rays. This is now known, 
however, not to be invariably the case, and the opinion 
is gaining ground that the ^- and 7-rays are not 
necessarily connected. The question of the real nature 
of the 7-rays was the last to be solved, and as the rays 
are not of primary importance at the present stage we 
may, with these experiments and remarks, defer the 
subject and pass on to the more detailed consideration 
of the two more important types of rays. 

The Feebly Penetrating k-Rays. 

Before proceeding to show experiments with the a-rays 
it is necessary to touch on certain considerations which 
come into play on account of their very great absorp- 
tion in passing through matter. In the first place, 
radioactivity is a mass or volume phenomenon. That 
is to say, every part, not the surface only but the inner 
portions also, of a radium salt, for example, is giving 


out a-, y8- and 7-rays. All these rays are absorbed 
by the substance itself very considerably, for the salts 
of radium are dense or heavy. But this absorption 
naturally does not affect the more penetrating rays 
nearly so much as the feebly penetrating a-rays. That 
part of the latter, generated inside the salt, does not 
escape at all. Only a very thin surface film contributes 
to the a-radiation. The consequence is that whereas, 
with the small quantities of radium that we have to 
work with, the strength of the penetrating rays is more 
or less proportional to the quantity of radium employed, 
with the a-rays this is no longer the case. The weight 
of the substance is less important than the amount of 
surface exposed. A very small quantity, say a milli- 
gram, of radium bromide, spread out as a thin film on a 
large plate, will give out immensely more a-rays than 
the same quantity in the form of a small crystal. In 
order to free the ^- and 7-rays from the a-rays, or the 
7-rays from the ;S-rays, it suffices to interpose screens of 
successively increasing thickness until the more easily 
stopped type is completely absorbed. But it is not 
possible so easily to eliminate by physical methods the 
/S- and 7-rays from the a-rays in order to leave the latter 
by themselves. For practical purposes, however, this 
result can be achieved very simply. If we take a very 
minute quantity of radium salt spread over a very large 
area, the ^- and 7-rays from so small a quantity will be 
so feeble as to be practically negligible, whereas the 
a-rays under these circumstances will reach their 
greatest intensity. For practical purposes a thin film 
of pure radium salt can be used to give a-rays by 
themselves, essentially free from yS- and 7-rays. 

Experiments with c-Rays. 

Such a thin film I have prepared for these experi- 
ments. On this shallow platinum dish, about a square 
inch in area, I have evaporated down a solution contain- 
ing about a milligram of pure radium bromide, and the 


dish, with its precious film open to the air, is carefully 
preserved when not in use in a special tube containing 
a desiccating agent to keep it dry, so that without undue 
risk of loss I can work with a bare film of radium salt 
and show you the a-rays. Over the bare film I bring 
the electrified silk tassel. It collapses instantly, in fact, 
much faster than it does when brought over the whole 
thirty milligrams of radium bromide contained in the 
mica-covered capsule. The a-rays from one milligram of 
radium produce more electrical effect than the 13- and 
7-rays from thirty milligrams. Now I cover the bare 
film of radium with a single sheet of thin writing-paper, 
which stops the a-rays completely, the /S- and 7-rays 
scarcely at all. You observe the tassel remains now 
charged as if the radium were absent. The /3- and 
7-rays from so small a quantity hardly appreciably 
discharge it. 

But if I displace the paper ever so slightly and 
expose a tiny part of the bare surface, the tassel instantly 
collapses. From these experiments, and the fact that 
it was the fashion at the time to cover radioactive sub- 
stances when experimenting with them, you will have 
no difficulty in understanding how it was that these 
feebly penetrating but intensely powerful a-rays re- 
mained at first neglected and almost unknown. 

The Range of c-Rays in Air. 

I now have to show you a very striking experiment 
indeed, suggested by some profound investigations of 
Professor Bragg in Adelaide, on the a-rays, to which we 
shall again have occasion to refer. So readily are these 
a-rays stopped that a few inches of air suffice entirely 
to absorb them. But the a-rays show this remarkable 
peculiarity not exhibited by any other type known. 
Each individual a-ray of any one homogeneous type 
travels exactly the same distance in an absorbing 
medium, and is stopped sharply and completely when a 
certain thickness of matter has been penetrated. The 

Fig. 9. — ArrARATUs to show Absorption of o-Rays by Air. 

Tig. II.— The SriNTHARiscoi'E of Sir William Crookes. 

To face p. 35 



consequence is that if we work with a homogeneous 
beam of a-rays, just without the distance of complete 
absorption, there is absolutely no effect, while just 
within there is a very large effect. I have said that the 
a-rays derived from radium are complex, consisting of 
four different types, each with a definite " range," as it 
is termed, or distance, it will travel in any given absorb- 
ing medium. For the purposes of this experiment, 
however, it is necessary to consider only the most 


Fig. 8. 

— ^PUMP 

penetrating type, which Bragg found could travel in 
air at atmospheric pressure and ordinary temperature, 
71 millimetres (or just under three inches) and no 
more. Now this flask (Figs. 8 and 9) is a little more 
than six inches in diameter, and it has been coated 
on the upper hemisphere of the inside surface with a 
phosphorescent film of zinc sulphide. For these a-rays 
the usual phosphorescers [e.g., the platinocyanides, 
willemite, etc.), employed for the /3- and 7-rays, are 
far less sensitive than crystallised zinc sulphide, or, as it 



is called, Sidot's hexagonal blende. The coated flask 
is arranged so that I can plunge my platinum dish with 
its bare radium film upward inside the flask and hold it 
centrally by a cork. In the dark, the flask being full 
of air, you observe hardly any glow. The three inches 
of air surrounding the radium film on all sides suffices 
completely to stop all the a-rays, and the /8- and 7-rays, 
from so small a quantity of radium, produce only a 
negligible effect on the zinc sulphide. But I have con- 
nected the flask to an air-pump and can pump out the 
air. At the very first stroke of the pump the whole 
globe flashes into luminescence, and as I continue 
pumping the glow gets stronger and fairly illuminates the 
immediate neighbourhood with its soft white light. I 
now readmit the air, and the glow disappears as suddenly 
as it came. So that you see, with somewhat carefully 
designed arrangements, and keeping in mind the peculiar 
properties of these a-rays which physicists have exactly 
worked out, it is possible even from a minute amount 
of pure radium bromide to obtain quite a fair amount 
of light, whereas the same quantity of radium less 
cunningly disposed would give very little effect. Radium 
compounds are usually preserved in sealed tubes so as 
to prevent them absorbing moisture from the atmo- 
sphere. Under these circumstances the effects produced 
by these a-rays are not observed. 

The Physical Nature of Radiation. 

Problems connected with the real physical nature of 
radiation are, it is well recognised, among the most 
fundamental in physics, and they involve more deeply 
perhaps than any others the great underlying meta- 
physical relationships between the external world of 
physical fact and the subjective mental processes by 
which we attempt to visualise these facts and obtain 
some sort of a reasonable explanation of them. Take, 
for example, the great problem that is always before us 
of the real nature of light. Is there anything more 


difficult of mental comprehension ? The difficulties 
are not minimised but rather increased by the very 
definite view we take to-day of energy as a separate 
entity having a real physical existence. 

Contemplate for a moment, if you can, the origin of 
the energy which impels every moving thing in earth or 
sea or sky. With the exception of a very small and 
practically negligible movement contributed by the 
tides and by volcanic agencies, and, it must not be for- 
gotten, by the radioactive substances themselves, all 
things which move do so directly or indirectly by virtue 
of the energy reaching this earth as radiations in the 
form of the sun's light and heat. Great masses move 
hither and thither here because of happenings at some 
time past, remote or recent, 90 millions of miles away in 
the sun. Inevitably, when we begin to contemplate 
radiation phenomena, we are driven to inquire into the 
medium filling the outer void of space by virtue of which 
this immaterial, but vital entity — energy — reaches us 
from far distant worlds. It is true we call it ether, 
and try to give to it all sorts of material, or pseudo- 
material, characteristics. Lord Kelvin seems to have 
spent a large part of his leisure time trying as it were to 
dematerialise matter into ether, that is, trying by all 
sorts of mechanically ingenious arrangements and 
analogy from material models — the only possible models 
our minds can yet grasp — to obtain a possible con- 
struction which would simulate the elusive but all per- 
vading ether. Others, on the well-known principle that 
topsy-turvydom, if only consistent and all-embracing 
enough, results finally in a system no less logical and 
rational than the original one, have given to the ether 
inconceivably great density, and to the atoms of matter 
the character of holes or voids in it. The necessity for 
the existence of a universal all-pervading medium, or 
ether, capable of transmitting energy, no one in these 
days of wireless telegraphy would deny, but on the 
question of its real nature opinion is as divided as it well 
could be. 


The tendency, however, in modern physics to-day is 
rather to derive and explain material phenomena from 
the properties of the ether than to attempt to construct 
an ether on a material or pseudo-material model. As 
yet, however, we know little about the properties of the 
ether itself. One definite thing we do know, for certain, 
and have known for a very long time, namely the 
velocity at which influences are transmitted across the 
ether. It is 185,000 miles a second, the speed of light. 
So far as we yet know, all influences that are transmitted 
by the ether travel at this one definite velocity. Not 
only light, but also the electro-magnetic radiations 
employed in wireless telegraphy, the magnetic storms, 
as they are termed, which reach us from the sun, and 
also, we believe, the X-rays, travel through the ether 
at this one definite speed. 

Corpuscular Radiation. 

The great mind of Newton two centuries ago ap- 
preciated to the full the fundamental difficulty in the 
explanation of radiation, and proposed the only way of 
escape from the more modern doctrine of an ether which, 
so far as I know, has ever been put forward. 

Light, on the Newtonian hypothesis, consisted in the 
emission from the glowing body of excessively minute 
material particles or corpuscles travelling with immense 
velocity. This corpuscular theory, so far as light is 
concerned, failed when subjected to a closer examination, 
and gave way to the present undulatory theory that light 
consists in a transverse vibration of the ether, the exist- 
ence of which, it was beginning to be recognised, was 
as great a necessity for the transmission of gravitational, 
magnetic, and other forms of energy which reach us 
from outer space as it was for the transmission of radia- 
tion itself. Though proved wrong so far as light is con- 
cerned, this idea of corpuscular radiation, strangely 
enough, will rank as one of the most suggestive flashes of 
Newton's genius, for it, in fact, anticipated by two 


centuries the march of experimental discovery. To-day, 
thanks to radioactivity, science has been enriched by the 
discovery of a-, y8-, and 7-rays, and two, at least, out of 
these types, the a- and the yS-rays, are not, like light, 
vibrations of the ether, but consist of the emission of 
excessively minute material particles (atoms and cor- 
puscles) travelling with immense velocity. This is one 
of two chief main lines of evidence that radioactivity is 
an accompanying manifestation of " atomic disinte- 

Into this aspect of the matter, however, I do not pro- 
pose entering to-night. Its consideration is more con- 
veniently deferred. It is sufficient to say that the a- and 
yS-rays, or, as I shall henceforth also refer to them, a- 
and y8-particles, comprise, the lighter fragments, as it 
were, of the disintegrating atoms of the radioactive sub- 
stance. In ordinary circumstances radium appears to 
be expelling both a- and /^-particles together, but this 
as we shall come to see is due to the fact that several 
successive disintegrations are occurring, and the effect is 
a composite one. The nature of these rays is so utterly 
different from that of light that it is worth while to stop 
and examine the difference a little more closely. 

The Wave Theory of Light. 

The wave theory of light has often been illustrated 
by what happens when a stone is dropped into a pool. 
Ripples extend outwards in concentric circles from the 
disturbance. The water, as the ripple reaches it, first 
rises above, then immediately afterwards falls below 
the normal level. The disturbance is propagated trans- 
versely, that is, outwards horizontally by a vertical, 
or up and down wave-movement of the water. The 
surface discloses the nature of the disturbance, but the 
same type of disturbance is taking place below the 
surface, and each circular ripple is in reality the section 
of a hemispherical shell. It is not possible to get an ether 
surface like a water surface, since the ether is all-per- 


vading. Light travels out from an incandescent point 
in all directions in spherical ripples, in which a to-and- 
fro motion of some kind is going on in the ether, trans- 
verse to the direction of propagation of the light. Con- 
trast with this what is believed to be the nature of the 
a- and /3-rays given out from a radioactive substance. 
The rays are given out uniformly in all directions, not as 
a succession of spherical waves, but as the random flight 
of immense swarms of tiny projectiles ejected from the 
radioactive substance. For shortness I shall call this 
the " discrete theory," as contrasted with the wave 
theory, because the radiation is considered to be due to 
the flight, radially outward from the substances like the 
spokes of a wheel, of swarms of free-flying, independent 
discrete particles. You could hardly imagine two more 
different phenomena, and yet that it is not easy to dis- 
tinguish between their effects is shown by the fact that 
for a long time a controversy raged between the two 
views regarding the nature of light itself. 

a- and is-rays due to the expulsion of 

I must anticipate a little here for the sake of clearness. 
It is now an old story that in the tiniest grain of matter 
there is a mentally inconceivable myriad of separate 
atoms. In this tiny quantity of radium bromide, 
weighing half a grain, we know with fair certainty there 
are fifty million billion (5 x lO-"^®) separate atoms 
of radium, assuming that the compound is pure. It 
has been proved that, roughly, one two-thousandth of 
these disintegrate yearly. There are about 32,000,000 
seconds in a year, so that in every second of time 
rather less than one thousand million of these radium 
atoms disintegrate, giving some small multiple of this 
number of a- and /3-particles. So mighty a host pro- 
jected outwards in all directions at random, as you may 
suppose, fill the surrounding space with their trajec- 
tories to all intents and purposes as completely as if 


they advanced as one continuous spherical wave-front. 
In other words, if only the number of projected particles 
is sufficiently great a discrete radiation will be, in many 
of its general effects and laws of propagation, not dif- 
ferent from a wave-radiation. It is true that such a 
radiation will show neither regular reflection, refrac- 
tion, nor polarisation in the manner that light does, 
and the absence of these phenomena for the a- and /3-rays 
is part of the evidence in favour of their discrete nature. 
If, however, we continuously reduce the number of 
particles ejected, in other words, if we continuously 
diminish the quantity of radium employed, there should 
come a point when the discrete radiation should no 
longer simulate the wave-type. It should, as it were, 
break up and show discontinuity, much as some of those 
faint continuous light-patches in the heavens, known 
as the planetary nebulae, when investigated by more 
and more powerful telescopes, begin to break up and 
show discontinuity, and finally are resolved into an in- 
numerable host of separate twinkling stars. Is it possible 
so to resolve a swarm of a-rays ? 

The Individual Atom of Matter. 

The older physicists who first deduced by accurate 
computation the weight and measure of the single indi- 
vidual atom and evaluated the number of billions con- 
tained in the smallest portion of matter perceptible to 
the senses, had they been soberly asked whether it 
would be possible ever to observe a single atom of matter, 
would have scouted the bare possibility. A single atom 
of matter ! A single atom of matter ! I recall this 
one exclamation, repeated over and over again with 
varying intonation by a distinguished foreign visitor, 
whose years had been spent at the microscope on the 
borderland between the perceptible and the imper- 
ceptible worlds, when the question we are now consider- 
ing was under discussion at a British Association 


Let us, however, now make a few calculations to see 
whether there is any hope whatever of being able to de- 
tect the effect of, say, a single a-particle expelled from 
radium, in the same sense as it has been found possible 
in astronomy to detect the individual stars which go to 
make up a planetary nebula. 

In an earlier lecture (p. 17) I alluded to the smallest 
quantity of radium that could be detected by the aid of 
the gold-leaf electroscope, that is, therefore, by means of 
the a-rays emitted. It was one three-thousand-millionth 
of a grain. Half a grain, as we have seen, gives out a 
few thousand million a-particles every second. So that 
the smallest quantity of radium detectable by the 
ordinary electroscope must be giving out only a few 
individual a-particles per second. From a very early 
stage it appeared not inconceivable to Rutherford that 
a discontinuity in the emission of a-rays might actually 
be detected by using a very minute quantity of radium. 

The Spinthariscope. 

The problem was actually solved, almost unawares, 
by Sir William Crookes, by means of an instrument he 
devised and called the Spinthariscope. The instrument 
is the only genuine instrument worked by radium that it 
is at present possible to buy at the optician's in the ordi- 
nary way, and it can be bought — radium and all — for a 
few shillings. The reason for this apparent paradox is 
to be found in the fact that it is in the essence of the 
result to be attained to reduce the amount of radium to 
the smallest possible quantity, and this unusual condi- 
tion allows of a practically unlimited number of spin- 
thariscopes to be made out of an almost invisible 
quantity of radium bromide. The amount of radium in 
each instrument is absolutely unweighable and invisible. 
A needle, A, is made to touch a tiny phial which once 
contained radium, and is then mounted (Figs. 10, and 11 
facing p. 35) centrally in a little brass tube, the size 
of a small reel of cotton, at the bottom of which is a 



phosphorescent screen, B, coated with zinc sulphide. 
At the other end of the tube is a lens, C, for magnifjdng 
the screen and, by means of a little screw, D, outside, 
the needle point may be moved nearer to or away from 
the screen. If now in a dark room the screen is ob- 
. served through the lens, it will be seen to be luminous, 
and this luminosity can be concentrated or spread out 
by screwing the needle point nearer to or farther from 
the screen. After the eye has become used to the dark- 
ness it will be seen that the luminosity is not just a quiet 

Fig. 10. 

continuous glow. The hght, like that of the planetary 
nebulae, has been resolved and shows discontinuity. 
It resembles most nearly a shower of shooting stars. 
Bright momentary flashes of light or scintillations, too 
numerous at any instant to count, are appearing and dis- 
appearing in the field of vision. These flashes are caused 
by the a-particles of radium. This minute insignifi- 
cant trace of radium is positively belching forth a- 
particles. It seems incredible that the incessant bom- 
bardment of the screen can be caused by such an in- 
finitesimal amount of radium. Yet so it is, and in a 
month's time, if the instrument is re-examined, it will 


be found that the scintillations are as numerous and as 
brilliant as formerly. After a time, perhaps a year, the 
phosphorescent screen itself will be worn out by the 
incessant bombardment, will become insensitive and 
need renewal. But replace it by a new one and the 
radium will be found to be as energetic as ever. The 
owner of the instrument will pass away, his heirs and 
successors, and even his race will probably have been 
forgotten before the radium shows any appreciable sign 
of exhaustion. 

The actual a-particle itself must, of course, be ex- 
tremely small. How else could a mere speck of radium 
send out such an incessant and numerous swarm ? As 
we have still to prove, the a-particle is an atom of helium, 
the second lightest atom of matter known. A grain of 
radium bromide expels every second about ten thousand 
million a-particles, and if we contemplate this mighty 
swarm expelled once every second of time throughout 
many centuries we may begin to have some idea of how 
many atoms there must be in a single grain of matter, 
and how small must be the single atom. The philoso- 
phers of only a generation ago would have ridiculed the 
hope that we should ever be able to look through a 
magnifying-glass to see the effect of a single atom of 
matter, yet each of the scintillations of the spinthari- 
scope is nothing else. 

Decay of c-Radiatign. 

The spinthariscope was the original, but to-day it is 
only one of many lines of evidence which have estab- 
lished the discrete character of the a-radiation and the 
nature of the a-particle. We know of many radioactive 
substances — polonium is one — emitting a-radiations, 
which gradually and completely lose their radio- 
activity with the lapse of time. Anticipating, we may 
say that the disintegration of polonium proceeds so 
rapidly that it is complete in the course of a few years. 
Were the process at all similar, for example, to the case 


of a hot body cooling, one would expect a gradual altera- 
tion in the character of the radiation with the diminu- 
tion of its intensity with lapse of time; whereas the 
character of the radiation is exactly the same at the end, 
when it has nearly all decayed, as it is at the beginning. 
This is explained simply on the view that the number 
of a-particles expelled grows less as the activity decays. 
The individual a-particles have the same velocity and 
other characteristics, whether expelled at the end or at 
the beginning of the process. Professor Bragg' s dis- 
covery that each a-particle has a definite " range," 
characteristic of it, is quite inexplicable on a wave 
theory. The range of the a-particles emitted by 
polonium, for example, is thirty-eight millimetres of 
air, and though in the course of a few years the a-radia- 
tion of polonium decays always completely, the range 
of the a-particle expelled at the end is exactly the same 
as at the beginning. 

Counting the a- Particles. 

In this connection, finally, I may mention some really 
wonderful work recently done by Professor Rutherford 
and his co-worker Dr. Geiger, in which they have actually 
succeeded in counting directly the number of a-particles 
expelled from a given quantity of radium every second. 
As you may know, if two points are connected to an 
electrical machine, or other method of generating 
an electric force or tension, a spark will pass between 
them under suitable circumstances. Now suppose the 
distance apart of the two points is just so great 
that no spark will pass with the particular electrical 
tension applied, and that some radium is then brought 
near to the points. Then a spark will pass. The rays 
from radium by making the air a conductor of electricity 
facilitate the passage of the spark, so that under their 
influence the discharge will leap across a greater distance 
than it otherwise would. Substitute for the crude 
method of detecting the discharge by means of a spark 



a highly refined electrical instrument, known as the 
electrometer, in which, as in the galvanometer, a spot 
of light is reflected from a mirror attached to a needle, 
which can be arranged to move when a discharge passes 
across the gap, and you have the essential principle 
of Rutherford's arrangement. Such an arrangement 
can be made so excessively sensitive that the passage 
of a single a-particle from radium through what cor- 
responded to the " spark gap " of the first arrange- 
ment described, is sufficient to cause the spot of light 
from the needle of the electrometer to move with a 
sudden jerk. The experiment consists, then, in counting 
the number of these sudden jerks of the electrometer 
needle in a given time, when a known quantity of radium 
is placed at a known distance. The radium has to be 
placed many yards away from the apparatus, and the a- 
rays are fired along a long exhausted tube with a small 
window at the end to admit the passage of a very minute 
definite proportion of the total number of a-particles, 
which proportion can be calculated. In the actual ex- 
periments the distance of the radium and the size of the 
window through which the «-particles passed were such 
that, roughly, only one out of every 100 million a- 
particles expelled found their way into the apparatus. 
The total number of a-particles actually expelled per 
second by a grain of radium in its normal condition was 
found to be about ten thousand million. Per milligram 
of radium the exact number per second is 136 million. 
These results were also checked by counting the number 
of scintillations per second in a special form of spinthari- 
scope. There have always been scientific men who have 
regarded the atom and the atomic theory with suspicion, 
and have never tired of insisting upon its " hypothet- 
ical " character. It may therefore be rightly regarded 
as one of the greatest triumphs of science that an ob- 
server can now actually sit down in front of a vessel and 
with the aid of a watch count the number of atoms 
entering it every minute from a quantity of radium 

di o 
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To face p. 47 



The y8-RAYS. 

In addition to their varying power of penetrating 
matter, there is another test which has proved of great 
service in analysing the three types of rays from radio- 
active bodies and in deterhiining the real nature of each. 
The trajectories of some of the rays are powerfully in- 
fluenced by a magnet, others are slightly, and others not 
at all affected. Thus the ;8-rays of all radioactive sub- 
stances if caused to traverse the space between the poles 
of a magnet are very strongly deflected, and if the magnet 
is a powerful one may be completely coiled up into closed 
circles or spirals. 

Faraday imagined that between the N-pole and S- 
pole of a magnet there existed actual lines of magnetic 
force. In the electro-magnet on the table (Fig. 12), 
which is formed so that the N- and S -poles are bent 
round so as to face one another, the lines of force between 
the opposite faces of the two pole-pieces are straight 
lines following the shortest distance between them. It is 
convenient to imagine with Faraday the actual existence 
of such lines of force. An electro-magnet is simply an 
arrangement in which a bar of soft iron can be magne- 
tised at will by passing an electric current through a coil 
of wire wound round it. Soft iron of good quality, unlike 
steel, retains no appreciable permanent magnetism. It 
is very easily magnetised by an electric current, and 
its magnetism continues just so long as the current, 
and ceases practically completely when the current is 
switched off. 



Deviation of ^S-Rays by a Magnet. 

Now suppose a beam of ;S-rays of radium to be fired 
through the space between the pole-pieces at right 
angles to the lines of magne ic force. The path of the 
rays is bent. The rays tend to coil round the magnetic 
lines of force in circles. Suppose we look along the lines 
of force stretching from the N-pole to the S-pole, that 
is to say, suppose the eye to be placed at the centre of the 
N-pole and to be looking towards the centre of the S-pole. 
Then the yS-rays will be coiled round into circles in a 
direction of rotation opposite to that of the hands of a 
clock, that is, as we say, counter clock- wise. If we look 
from the S-pole to the N-pole the direction of rotation is 
clock- wise. Now if the radium is placed behind the poles 
of the electro-magnet, and a screen of platinocyanide of 
barium is placed in front, and the distance between them 
is so adjusted to the strength of the magnet that when the 
latter is excited by an electric current the /3-rays from 
the radium are coiled up into circles of lesser diameter 
than the distance between the radium and the screen, 
none of the /3-rays will now reach the screen. This will 
be seen from Fig. 13. In this figure the eye is supposed 
to be at the centre of the S-pole of the magnet, looking 
towards the face of the N-pole. The rays from the 
radium passing up between the N-pole and the eye, 
in the top diagram, reach the screen. In the lower 
diagram the magnet is in action, and the rays are coiled 
clock-wise into circles, none reaching the screen. 

The radium is contained in its mica- covered capsule 
so that only the ^- and 7-rays are dealt with, the a-rays 
being suppressed. In the darkness you see the phosphor- 
escent screen brilliantly luminous so long as the magnet 
is not excited. I switch on the current and the light of 
the screen at once goes out almost completely. The 
faint luminosity left behind is due to the 7-rays, which 
are not deviated at all, so far as we know, even by the 
strongest magnetic forces. If I interpose a penny in 


front of the radium so that the 7-rays have now to 
traverse it before reaching the screen the faint lumin- 
osity is hardly diminished. Now I switch off the excit- 
ing current and the magnet almost instantly loses its 
magnetism, the /3-rays spring back out of their circular 
into straight trajectories, strike the screen and cause it 


Magnet off. 


Magnet on. 
Fig. 13. 

again to flash out into brilliance. Now the introduc- 
tion of a penny causes the luminosity practically to 
disappear, all but for the faint glow due to the 7-rays. 

Electric Charge carried by /S-Rays. 

To a trained physicist the interest of this behaviour 
is due to the fact that it is exactly what would happen 
to a current of electricity if it were made to flow between 


the poles of a strong electro-magnet. If we employed 
a piece of ordinary wire to carry the current, the wire 
would tend to coil up into a circle exactly like the /S-ray, 
and there would be a battle between the natural stiffness 
of the wire and the deviating magnetic force, and it 
would depend on their relative strengths which prevailed. 
With care, however, it is possible to use a fluid wire, 
which has no stiffness. If a strong current is passed 
through a thin aluminium wire it, of course, gets hot 
and finally melts, but retains its original form without 
breaking, hanging by virtue of its weight as a beautiful 
loop of glowing molten aluminium. Such a loop pro- 
vides an extremely sensitive means of investigating the 
laws of action of magnets on currents, and you can see 
how violently and powerfully it is deviated if it is hung 
between the poles of the electro-magnet and the magnet 
then excited. The iS-rays, as they traverse their course, 
behave exactly like a current of electricity. If they con- 
sisted of extremely rapidly moving particles — charged 
with electricity — we know that such a stream would 
behave to a magnet exactly like a current flowing in a 
flexible conductor. 

The Nature of Electricity. 

Now we do know the direction in which the ^-rays 
are moving, namely from the radium, but we do not 
know, or at least did not till recently know, the direc- 
tion in which the electricity is moving in an electric 
current. However, by long usage we speak in a purely 
conventional way of the + and - ends of a wire in 
which a current is flowing. We do not yet know 
for certain whether there are two kinds of electricity, 
a positive kind and a negative kind, but the probability 
is that there is only one kind, the negative kind, and that 
the effects of the opposite kind are due to a relative 
electrical scarcity or vacuum. It is much the same with 
heat and cold, except that we know the real thing is 
heat, and cold is the absence of it. A trained physicist 


will speak of so much heat, or so little heat, or or one 
body having so much less heat than another, but he will 
not speak of so much cold, or one body having more 
cold than another, although often such a method of 
expression would be convenient and would lead to no 
error. In this sense we may speak both of positive 
and negative electricity without error. A current of 
electricity flowing along a wire from the positive to the 
negative we may look upon as due to the transport of 
positive electricity in the direction from + to -, or as 
the transport of negative electricity from - to !- . The 
two ideas are equivalent and, in fact, identical for the 
present purposes. On the view that there is only one 
kind — the negative kind — of electricity, a positively 
charged body or atom is merely a body or atom with less 
negative electricity than is normally present in an " un- 
charged " or electrically neutral body. 

In the /S-rays we have a movement of charged par- 
ticles /rom the radium, and we have to find out whether 
the particles are positively or negatively charged, using 
the terms positive and negative in their conventional 
electrical significance. If the rays were deviated in the 
same sense as a current flowing from + to - in the same 
direction as the rays, obviously we should conclude the 
/3-rays were + ly charged. As a matter of fact we 
find the opposite is the case. When the yS-rays 
are deviated clock-wise by the magnet a current of the 
kind described would be deviated counter clock-wise. 
To simulate the deviation of the /S-rays the electric 
current must be a negative current, that is to say, must 
be either negative electricity flowing in the direction of 
the rays, or positive electricity flowing in the opposite 
direction. As there is no reason to doubt that the 
y8-rays do come from the radium, the electric charge 
they carry must be negative. 

Modern views are definite on the point that if there is 
only one electricity, that one is the kind which by con- 
vention has, unfortunately, been styled negative. The 
negative is the real electricity. The positive may, like 


cold, be the mere deficit of the real kind, or it may have 
a separate existence, the mirror image as it were of the 
other kind. I personally have always preferred the 
view that negative electricity is " electrical heat " and 
positive electricity " electrical cold," but a real answer 
to this question would no doubt prove itself to be a very 
fundamental step, and would require much further con- 

The behaviour of the /3-rays in a magnetic field as- 
sociates them at once with some previously known 
radiations from the electrical discharge tubes exhausted 
to an extremely high degree of vacuum which are known 
generically as Crookes' tubes, from their first systematic 
investigator. Into this field of work I have no intention 
of entering in detail, for it is the one aspect of this subject 
which has received the most adequate treatment in the 
accounts of radioactivity written for the benefit of the 
public. A brief resume only must suffice. 

Radiant Matter or Cathode-Rays. 

The ^-rays are very similar in nature to the " Radiant 
Matter " (also called " cathode-rays " or " cathode- 
streams ") of Sir William Crookes, obtained when an 
electric discharge or current is passed through a vessel 
exhausted to a very high degree of vacuum. The requi- 
site degree of vacuum can be obtained with a little trouble 
by the aid of a mercury pump, based on the same 
principle as Torricelli's celebrated experiment with the 
barometer. But far quicker and more efficient methods 
have lately come into use. One such consists in absorb- 
ing the last traces of gas in the pores of the charcoal of 
cocoa-nuts cooled to the temperature of liquid air, 
according to the discovery of Sir James Dewar. 
Another method consists in absorbing the last traces of 
gas with the vapour of metallic calcium heated to a very 
high temperature in a special vacuum furnace. The 
discharge from the cathode, or negative pole, in a high 
vacuum then consists of radiant streams of particles 


travelling in straight lines and producing vivid green 
phosphorescence where they strike the glass walls of the 
vessel. Any obstacle placed in their path casts a sharp 
shadow, the glass beyond not fluorescing where pro- 
tected from the bombardment by the obstacle. These 
particles also carry charges of negative electricity, and 
have great energy, heating to whiteness a piece of 
platinum interposed in their path, and causing the most 
intense fluorescence of willemite in the same way as the 
radium rays. Like the /5-rays they are deviated by a 
magnet, and in the same sense, only very much more 
easily. Here is a form (Fig. 14) of Crookes' tube, 
designed to show the cathode-rays and their deviation 
by the action of a magnet. The electrodes consist of plates 
of metal, which are attached to the terminals of an 
induction coil, or an electrical machine or other suffi- 
ciently powerful source of electric tension. One elec- 
trode A is connected to the positive pole, and the other 
electrode B to the negative pole of the coil, and we have 
to concentrate our attention on the negative electrode, 
this being what is also called the " cathode." The tube 
has been exhausted by a pump until there is only about 
one ten-thousandth of the air left, and was then sealed 
up. Under such conditions the glass of the tube shines 
with a brilliant fluorescence when a discharge is forced 
through it. This fluorescence has been traced to " rays " 
inside the vessel, proceeding from the cathode at right 
angles to its surface and travelling in straight lines 
through the tube. Wherever they strike the glass they 
cause it to glow, just as the radium rays do. 

In front of the cathode is a piece of mica with a slit cut 
in it, which stops all the rays except a narrow pencil 
passing through the slit. Along the length of the tube is 
fixed a fluorescent screen in the form of a plate painted 
with powdered willemite, and as the narrow pencil of 
rays impinge on this plate they trace out their path as a 
bright line of green fluorescence. Now if one pole of a 
magnet is brought behind the tube the rays are bent 
sharply to the left or right, depending on whether the 





Fig. 14. 


N- or the S-pole of the magnet is presented to the tube. 
The direction of the deviation is the same as with the 
/3-rays, and before even the ;g-rays had been discovered 
the cathode rays of the Crookes' tube had been definitely 
shown to consist of minute particles charged with 
negative electricity flying off from the cathode with 
immense velocity. 

The Electron. 

What are these particles ? Crookes thought they 
were matter in a new or fourth state. To-day we know 
they are " electrons." The electron is a new and some- 
what startling conception to minds trained on the older 
lines, although traces of it date back from the dis- 
coveries of Faraday of' the laws of electrolysis. We 
owe largely to the well-known investigations of Sir 
Joseph Thomson, and his school at the Cavendish 
Laboratory, Cambridge, the recognition of the electron 
as an atom of electricity, divorced from matter. The 
cathode-rays consist of these separate individual and 
isolated electrons, repelled out of the metal of the nega- 
tive pole under the action of powerful electric stress, 
and, in the absence of gas, gathering terrific speed in 
their passage through the exhausted tube. Whatever 
the manner in which these electrons are produced, 
under whatever circumstances they result, they are 
always identical in their main characteristics. Their 
charge is always the same, and also their " mass," 
although their velocity may and does vary according 
to the conditions within very wide limits. They and 
their motion are responsible for the most varied and 
apparently unconnected phenomena in Nature, and in 
the empire of matter they seem often to occupy a r61e in 
comparison with the more massive material a':oms 
analogous to the part played by the planets in relation 
to the central sun of a solar system. The mass of the 
electron is only one two-thousandth part of that of the 
hydrogen atom, the smallest particle previously known. 

The methods employed depend upon tracing the 


paths of the cathode-rays when they were subjected 
simultaneously to electric and to magnetic fields. 
Both fields deflect the cathode-particle but in different 
ways, and from the results the charge, mass, and 
velocity of the particle were separately found. 

Inertia or Mass. 

In some ways we know far more about the electron 
than about the atom of matter. The electron cannot 
move without disturbing the medium which occupies all 
space continuously, and which we, not yet knowing too 
much about its real nature, call the ether. It is the 
motion and change of motion of the electron which give 
us light, the X-rays, and the long ether waves used in 
wireless telegraphy. It is the reaction of the ether on 
the moving electron which gives it its " mass." Now 
this " mass " of the electron, applied as the term was to 
the atom of pure electricity entirely unassociated with 
matter, needed very careful and clear thinking, or it 
would appear utterly contradictory to the older concep- 
tions of matter. The term mass, used in this sense, has 
nothing to do with the effect of gravity or weight, as it 
is still absolutely unknown whether electrons obey the 
law of gravitation. 

In this region of new ideas we are now entering, more 
difficulty, perhaps, is to be anticipated in the meaning 
attached to the terms employed than in the actual ideas 
themselves. Mass is often equivalent to " weight," but 
here it is not so. The mass of, meaning the quantity of, 
matter, is a fundamental idea, while weight is a derived 
idea due to the earth's attraction. A given quantity of 
matter throughout the universe has an unchanging mass. 
Its weight, of course, depends upon the proximity and 
ma ;s of the world attracting it. What then is the measure 
of mass as distinct from weight ? Weight is, as a matter 
of fact, invariably used on the earth to measure mass 
because it is so convenient. Yet if we can imagine our- 
selves isolated in space at a great distance from all worlds 


with a given quantity of matter it is desired to know the 
the mass of, we should still have no difficulty in dis- 
tinguishing the greater mass, say of a sphere of lead, 
from the lesser mass of a similar- sized sphere of wood. 
We should know the difference by the difference of 
inertia. If we struck each a similar blow the wood 
sphere would start to move many times as fast as the 
lead sphere. Neither would have appreciable weight 
under these circumstances, but their relative inertia 
would still be in proportion to their masses. A collision 
between two " weightless " railway trains meeting in 
mid-space would work just as much havoc to the trains 
as it would if it occurred at the same speed upon the 
earth. Hence when a physicist speaks of the " mass " 
of a/3-ray particle, or of a cathode-ray particle, no con- 
siderations of weight are in his mind. 

Sir J. J. Thomson, first with these cathode-rays, after- 
wards with the /3-rays, showed how it was possible, by 
measuring the extent to which they were deviated by 
magnetic and by electric forces, to determine the 
velocity, the charge, and the mass of the particles which 
constitute them. 

The application of these methods resulted in the proof 
that the charge and the mass of the ;8-particle were 
identical with that of the cathode-ray particle of vacuum 
tubes, but the velocity of the iS-particle was far higher 
than that of the fastest known cathode-ray. Thus the 
/3-particle ejected from the radium atom was already 
known. It is true it is ejected more violently by radium 
than in any previously known case, but in its essential 
characteristics, its charge, or the quantity of electricity 
it carries, and its mass — it is the same particle as Sir 
William Crookes dealt with in his vacuum tubes thirty 
years ago. He christened them in a prophetic moment 
with the name of " Radiant Matter," and was, like many 
another prophet, ridiculed for his pains. 


Velocity of the ^-Rays. 

The cathode- ray particle, and also the y8-ray particle, 
were found to carry the same amount of electricity as 
the charged hydrogen atom. Hence, whatever else the 
/3-particle of radium is, it is certainly an atom of nega- 
tive electricity. With regard to the velocity, just as the 
mass of these particles is smaller than any known 
material particle, their velocity is appropriately almost 
inconceivably greater than that of any previously known 
material particle. It approaches that of light itself, 
which has a velocity of 185,000 miles per second. The 
average velocity of the cathode -ray particle of the 
vacuum tube is from 5,000 to 10,000 miles per second ; 
while that of the fastest of the /3-particles of radium is 
so nearly that of light as to be indistinguishable from it. 
Most of the /3-rays, however, travel with a velocity from 
40 to 80 per cent, of that of light. 

This is one of the most general, as it is one of the 
most remarkable, features about radium. The effects 
produced by its rays, even the rays themselves in some 
part, are not entirely new. They can be simulated to 
some extent by artificial means. In passing from the 
effects produced artificially to those produced by radium 
spontaneously, we are aware of great resemblances, and 
at the same time of great differences. By the use of 
exceedingly powerful electrical appliances, and the ex- 
penditure of a considerable amount of energy, we can 
simulate to some extent the ;8-rays of radium, but no 
instrument maker at the present time can provide you 
with the means of impressing upon the artificially 
generated cathode -ray electron of the Crookes' tube more 
than a small fraction of the velocity with which the 
/3-ray electron is being spontaneously expelled from 
radium. It is the same in other matters. The utmost 
we are able to effect by the most powerful forces at our 
disposal falls far short of what is being done spon- 
taneously by a mere speck of matter undergoing atomic 



The Radium Clock. 

Before leaving the subject of ^g-rays, I have to show 
you an interesting instrument devised by Professor 
Strutt/ and popularly called the radium clock 
(Fig. 15). It is the nearest approach to perpetual 
motion that has yet been devised, and it consists of a 
gold-leaf electroscope, worked by the 
negative electricity carried away from 
the radium by means of the ^-rays. 
A few milligrams of a salt of radium 
are contained in a thin-walled closed 
glass tube, A, through which the 
j8-rays can easily penetrate, and this 
tube is supported from an insulating 
rod of quartz, B, within a highly 
exhausted glass vessel. The tube in 
turn carries at its lower end two gold 
leaves, C, after the manner of an 
electroscope. The yS-rays shot out 
from the radium carry away negative 
electricity, and therefore the radium 
itself left behind becomes positively 
charged. The gradual accumulation 
of this charge causes the gold leaves 
attached to the tube to diverge little 
by little, until they touch the sides of 
the vessel and are discharged, when the 
cycle of operations recommences. The 
instrument on the table (Fig. 16, 
facing p. 47) was constructed many 
years ago, and has been functionating about once every 
three minutes ever since. There is no reason why it 
should not do so for at least a thousand years more, 
though at a slowly decreasing rate. Though not a true 
perpetual motion machine, it is one so far as only our 
lives are concerned. 

Fig. 15. 

Now Lord Rayleigh. 


Magnetic Deviation of a-PARTicLES. 

The methods we have been considering which led to 
the elucidation of the real nature of the iS-rays — the 
determination of the nature of the expelled particle, its 
mass, charge, and velocity — have been applied success- 
fully also to the elucidation of the real nature of the 
a-rays, though here the task was very much more 
difficult experimentally. Rutherford, to whom we 
owe our knowledge of this subject, worked for a long 
time before he could detect any influence produced 
by the most powerful magnets on the course of the 
a-rays, so slight and insignificant it is compared with 
the effect on the ^-rays. Finally, he proved that the 
a-rays were deflected both by electric and by magnetic 
forces, but to an extent of the order of one-thousandth 
part of the effect that the /3-rays suffer under similar 
circumstances. The deviation of the a-rays, moreover, 
is in the opposite direction to that of the ^S-rays. Where 
the yS-rays are coiled clock-wise, for example, the 
a-rays would tend to turn counter clock-wise. By 
these, and numerous other experiments, it has been 
shown that the a-rays consist of positively charged 
particles. The a-particle is, however, not, like the 
yS-particle, only a disembodied electrical charge. It is 
a charged material atom. At first it was thought to 
be twice as heavy as the hydrogen atom, on the assump- 
tion that it was charged with a single " atom " of positive 
electricity. Now, however, it has been proved to carry 
two charges of positive electricity, and to be an atom 
four times as heavy as hydrogen. This is in accord with 
the whole of the rest of the evidence of radioactive 
changes still to be considered, which points unmistakably, 
though indirectly, to the conclusion that the a-particle 
is an atom of the element helium. The atomic weight 
of helium is four, or, in other words, the helium atom 
is four times as massive as the hydrogen atom, which 
is always taken as unity. In our most recent view, 


to be later considered, the a-particle is the nucleus 
of a helium atom that has lost the two electrons that 
accompany it as satellites in the normal " uncharged " 

Velocity of the a-PARTiCLE. 

Waiving the case of the /3-rays which, as we have 
seen, are electrical rather than material in nature, the 
a-rays of the radioactive substances furnish without 
doubt one of the most wonderful phenomena at present 
known. If radium did nothing else but send out these 
a-particles, that alone would of itself constitute a new 
epoch in our knowledge of nature. Take their velocity, 
for instance, which, though lower on the average than that 
of the ^-rays, reaches in spme cases the very handsome 
value of over 12,000 miles per second. This is hundreds 
of times faster than the next fastest known material 
thing moving in earth or air or space. The swiftest 
flight known previously is that of some of the shooting 
stars, which attain sometimes to a speed of from twenty 
to forty miles a second, and from the attack of which we 
are largely protected by the fact that their velocity is so 
great that they are quickly dissipated into vapour by 
the simple resistance of the air. While such a meteor 
was traversing the distance to the moon an a-particle 
would, given an unimpeded path, reach the sun. 

Such a velocity multiplied by itself, or squared, gives 
us a measure of the energy possessed by the a-particles. 
If their velocity is, say, half a thousand times faster 
than any previously known, the kinetic energy they 
possess is, mass for mass, a quarter of a million times 
greater than any we have ever had to do with before. 
In this fact lies the key to many of the surprising revela- 
tions of radium. When we speak of being able to detect 
the effect of a single a-particle, and therefore of a single 
atom of matter, we mean the detection of its energy, 
which is a quarter of a million times as great as that of 
any other kind of atom known to us. Similarly, when we 
speak of being able to detect in a few seconds by radio- 


active methods the course of a change which would have 
to proceed continuously for geological epochs before it 
produced an effect detectable by the most sensitive 
chemical test, it is because, firstly, we detect the energy 
evolved by the change, not the change itself ; and, 
secondly, because the energy is at once so relatively 
enormous and at the same time so much more easily 
detected compared with any other kind of energy out- 
burst previously known to us. 

Passage of cu-Particles through Matter. 

Matter moving with the speed of 10,000 miles a second 
is so novel and strange to us at present that it is doubtful 
whether our ordinary conceptions afford much guide or 
analogy. The muzzle-velocity of a cannon-ball, for 
instance, is a small fraction of one mile per second. Now 
we have seen that the a-particle of radium is capable of 
traversing very thin aluminium leaves and also several 
inches of gaseous air. It is extremely interesting to 
inquire what happens during the collision of an a-particle 
with a molecule of gas or metal. Some at least of 
these collisions must be full and direct, not simple 
grazing or glancing coincidence ; and it seems at first 
sight difficult to believe that an a-particle striking a gas- 
molecule full and fair should not be stopped, however 
fast it is moving. Nevertheless, it is not so. Upon this 
matter the researches of Bragg and his colleagues have 
thrown a flood of light. His conclusions are as remark- 
able as they are definite. " Each a-particle pursues a 
rectilinear course, no matter what it encounters ; it 
passes through all the atoms it meets, whether they form 
part of a solid or a gas (or, in all probability, of a liquid), 
suffering no deflection on account of any encounter until, 
at any rate, very near the end of its course. ... A thin 
metal plate may be placed in the way of the stream, 
and so rob every particle of some of its energy, but not 
a single one is brought to rest by collision with the atoms 
of the metal, and the number of particles in the stream 


remains unchanged." Surely this vivid picture of the 
flight of a swarm of a-particles raises anew the old meta- 
physical conundrum of the schoolmen, whether two 
portions of matter could occupy the same space at the 
same time. For the only possible meaning of Professor 
Bragg' s conclusion is that the a-particle must go clean 
through the atoms of matter it penetrates as though they 
were not there, and therefore at the instant of collision 
the two atoms do occupy the same space at the same 
time. This power of the interpenetration of masses is one 
of the peculiar properties of matter moving at these, 
what may be termed ultra-material, velocities. We 
know for certain it is not a normal property of matter. 
The only apparent consequence of the passage of the 
a-particle through the atoms it encounters is that it 
ionises them, that is, they become charged, some with 
+ and some with - electricity, after the collision. It is 
probable that the a-particle possesses its charge when it 
is expelled from the atom. But whereas in the case of 
the ^-particle the charge of electricity is the particle, in 
the case of the a-particle the charge would almost 
certainly result as a consequence of the velocity with 
which the particle is moving, even if it were uncharged 
initially. At least it is certain that no atom moving at 
10,000 miles a second would continue uncharged. The 
very first collision with an atom of matter would " knock 
out an electron or two," that is to say, charge the 
moving particle positively. 

Scattering of a-PARTiCLES, 

Since the above quotation was written by Professor 
Bragg it has been proved that some of the a-particles do 
suffer a deflection or scattering in their passage through 
matter. For the vast majority of the a-particles this 
deflection is exceedingly slight, but for a very small 
proportion of the whole the deflection may be so great as 
practically to turn the a-particle back the way it came. 
This is extremely interesting. The a-particles alone 


have access to the real interior of the atom of matter, 
and a close study of this phenomenon has resulted in 
information being obtained as to what the atoms of 
matter consist of. Hitherto science has been completely 
confined to the external characteristics of atoms, but 
the a-particles, after their passage through these atoms, 
will afford some clue, which will be later considered, as 
to the nature of the unknown territory which they have 

The quotation from Professor Bragg (p. 63) pursued 
the question of what happens to the a-particle on collision 
only as far as the initial stages. Each atom of matter 
penetrated robs the a-particle of some of its energy, and 
its velocity is therefore diminished as it pursues its path. 
But the more slowly it moves the more energy is with- 
drawn from it in passing through any given obstacle. 
In addition, the slower it moves the more easily is it 
deviated from its course, or scattered. In consequence, 
the speed is more and more quickly reduced as the end of 
its path is approached, and the a-particle thus passes out 
of the range of detection somewhat suddenly. 

A Method of rendering the Tracks of 
k-Rays Visible. 

By an ingenious arrangement, C. T. R. Wilson 
has succeeded recently in making the paths of many 
of the new radiations in air, or other gas, visible to the 
eye, and in actually photographing them. These rays 
ionise the gas, and leave in their tracks columns of ions, 
which are molecules of the gas carrying an electric 
charge, and which, although really moving about like 
all gaseous molecules at great speed, are, by comparison 
with the much swifter rays producing them, almost at 
rest. Now these ions, the negative variety more easily 
than the positive ions, afford nuclei for the condensation 
of moisture from a supersaturated atmosphere. Dust 
plays the same part, but all dust can readily be removed. 
When moist air in a closed space is suddenly expanded 

Fig. 17. 

Fig iS. 

Fig. iq. 
Cloud-Tracks of k-Rays of Radium. 

To face p. 6s 


the air is cooled and the moisture condenses as mist or 
rain on the dust particles^ and carries them down, so 
freeing the air from such impurities. If the pure air 
is now suddenly expanded within certain well-defined 
limits, in the absence of ions or dust, no condensation 
is produced. But if the air is traversed by any of the 
new ionising radiations, the tracks of the rays, when the 
ionisation chamber is suitably illuminated, appear 
momentarily as long spider-threads of mist whenever 
the air is suddenly expanded and chilled. If a flash of 
light is arranged to take place just after the expansion, 
the threads may be photographed. In Fig. 17 is shown 
the a-rays proceeding from the needle point of a 
spinthariscope (p. 43), and in Fig. 18, in the lower part 
of the picture, an enlargement of the end of the track 
of a single «-particle. 

The tracks left by the a-particles are almost all 
perfectly straight, but a very few show abrupt 
large deflections, and sometimes actually the direc- 
tion of travel is nearly reversed. The yS-rays, on the 
other hand, give very zigzag tracks. These rays- 
are known to be scattered and turned very readily 
by their encounter with the molecules of matter, 
and owing to the ionisation they produce being less 
intense than in the case of the a-rays, their tracks 
are much fainter. In the upper part of Fig. 18 is seen 
the end of the track of a ^-particle, just before it stops 
and ceases to ionise. At first when the yS-particle is 
travelling at high velocity its track, which in air may 
be several metres long, is very much straighter, and it 
may travel for several centimetres without sensible 
deflection. The photograph has caught the end of the 
track when its energy is feeblest and its liability to be 
deviated greatest. In Fig. 19 are shown the photographs 
of two a-ray tracks, one the normal practically straight 
path, and the other showing two abrupt changes of 
direction in its length. To quote C. T. R. Wilson's own 
words: "The a-particle has thousands of encounters 
with atoms of the gases of the air in each millimetre of 


its course by which ionisation is brought about, as we 
know from measurements made by the electrical method, 
and in accordance with this the cloud particles (which 
are simply ions magnified by condensation of water) 
are so closely packed that they are not separately 
visible in the photograph. It is remarkable that only 
two encounters out of the many thousands occurring in 
the course of its flight should succeed in deviating the 
particle visibly from its course and that in these cases 
the deviations should be quite large." We shall have 
occasion to refer to this again as these phenomena have 
thrown much light on the internal structure of atoms. 
The experiments have also thrown light on the nature 
of the 7-rays, and have made it appear probable that 
these rays do not ionise the gas directly, but first 
cause the molecules struck to emit a kind of cathode- 
or /3-radiation, and it is these secondary radiations 
which produce the ionisation. 

The Fate of the a-PAHTiCLE. 

Fluorescent, photographical, and electrical actions 
all cease simultaneously. It is estimated that at the 
moment the «-particle ceases to be detectable it is still 
moving with the velocity of several thousand miles a 
second. For all that is known the particle may then 
suffer a sudden stop, or it may continue its course 
without ionising the atoms it encounters. 

For us who are concerned, for the most part with the 
broad limitations of our past and present knowledge, the 
most interesting feature of this phenomenon is that it 
indicates quite definitely that an a-particle expelled 
with an initial velocity below several thousand miles a 
second could not by any of the present known methods 
be detected. Any of the apparently stable and non- 
radioactive elements might be disintegrating and 
expelling a-particles, but if these did not attain this 
limiting speed we should have no evidence of the fact. 
It is really by a somewhat slender margin of velocity 


that the a-particles have come within our knowledge 
at all. The light we have gained has but served to 
intensify the darkness by which we are surrounded on 
all sides. Processes similar to and but little less energetic 
than those which produce radioactivity, may be going 
on suspected everywhere around us, without producing 
any yet detectable effects. Radioactivity is to be re- 
garded rather as a benevolent hint given to us by Nature 
into secrets we might never have guessed, rather than as 
the necessary and invariable concomitant of the processes 
of atomic disintegration. 



The Source of Radioactive Energy. 

If we are to continue to regard energy in the modern 
way as something having a definite existence, we have 
to answer the question, " From where does the energy 
of radium come ?" That it comes from nowhere, or 
that it is being newly created out of nothing by radium, 
is a view it is not possible to entertain for a moment with- 
out destroying the basis upon which nineteenth-century 
physical science has largely been reared. ''How has it 
got the energy in it to do it ?" is the first question that 
naturally arises in the mind with regard to radium, but 
obviously we should first ask, " Has it the energy in it ?" 

Two Alternative Theories. 

If the doctrine of energy is true, there are fortunately 
only two possible alternatives to be considered. Either 
the energy must be derived from within the radium, 
which we shall call the first, and as we think the true, 
alternative, or it must be supplied from outside the 
radium, and this we shall call the second alternative. 
This simple narrowing down of all the possible issues to 
two alternatives may appear to you somewhat trite, but 
in reality it carries with it far more than appears on the 
surface. In the first place, being an intrinsic property 
of the element, radioactivity is therefore a property of 
the atom, and if we take the first alternative and say 
the energy comes from within, it means from within the 
atom, and therefore that there must exist an enormous 



and not previously suspected store of energy in matter, 
or at least in radioactive matter, in some way inside its 
atoms or smallest integral parts. 

On the second alternative, which has often been 
advanced, radium acts merely as a transforming 
mechanism. There are electrical transformers dotted 
all over this city, receiving the economically transmitted 
but dangerous high-tension currents from the central 
power station and delivering the comparatively safe low- 
tension currents to your houses, which are wasteful to 
transmit for long distances. Are the atoms of radium 
acting as the transformers of a mysterious and hitherto 
unknown source of external energy, first receiving it and 
then delivering it up again in a form which can be recog- 
nised ? It may be said at once that so vague a view, 
postulating the existence of illimitable and mysterious 
supplies of energy from without, cannot be directly 
disproved. At first it seemed to provide a way of 
escape from some of the more unpalatable logical con- 
sequences of the first alternative and was eagerly 
adopted. In reality, instead of a way of escape, it 
proves to be a veritable will-o'-the-Mdsp, luring on its 
followers beyond the limits of credulity into a quagmire 
of unsubstantial hypotheses, so bottomless and unreal 
that even the facts of radium are a wholly inadequate 
justification, and, even so, incapable of throwing any 
light on the facts when these are more nearly examined. 
Nevertheless, we must pursue both alternatives im- 
partially, if only to leave no doubt that both have only 
to be fairly considered for one to be dismissed. 

On the second alternative the radium owes its activity 
to a supply of energy from outside. One has only to 
isolate the transformers which light this city from all 
connection with the outside central station to plunge the 
city in darkness. But we have seen that to quench 
radioactivity or to modify it in any way is one of the 
things science cannot do. Experiment has proved that 
even in the natural state in the mine, hundreds of feet 
deep down in the earth, pitchblende exhibits its normal 


radioactivity. So that if it derives its energy from 
"s^-ithoiit. this must be of a kind entirely different from 
any at present known, for it must be capable of travers- 
ing A^-ithout loss hundreds of feet of solid rock. This is 
as far as we need pursue the second alternative for the 
moment. Provided we can call into existence a new 
kind of radiant energy luilimited in amount, permeating 
all space and unimpeded by passage through matter of 
any thickness, we may, but only so far as we have yet 
gone, seek a bare explanation of the energ%' of radium on 
the second alternative. Such a xievr would accord at 
first sight with the continuous and permanent acti\-ity of 
radium for an indefinite time, and there would be no 
reason why radioactivity, however intense and power- 
ful, should decay or diminish with the lapse of 

But if the first alternative is true, and the energ\' 
comes from within, large as the store of enevgy in the 
atom must be to explain radioactivity, it cannot be 
infinite, and therefore it is to be expected that the 
activity will slowly decay with the lapse of time. If 
two radioactive bodies, one much more powerfully 
radioactive than the other, are compared together, it 
is to be expected on this xievr that the acti%ity of the 
more powerful body will decay faster than that of the 
other. But for both a time will come, as soon as 
the internal stores of energA* are exhausted, when the 
radioactivity will come to an end. 

By far the most important consequence of the first 
alternative, however, has still to be considered. Radium, 
if we call by that name the substance containing the un- 
evolved store of energy, can no longer be radium when 
the energy- is lost. Coal is not coal after it is burnt. 
When energ\-is obtained from matter the matter changes, 
and before it can be regained in its former state the 
energy evolved must be put back. In no case is it 
possible for matter to part with its store of energy and 
remain the same, for otherwise you wiU readily see a 
perpetual motion machine would be easy enough to 


construct. Indeed, most of those attempted involved 
this impossible assumption. 

But we have seen that if the energy is stored up in 
the radium it must be -mthin the atom., and, therefore, 
if radium changes, it must be a change of the atom and 
of the element itself. This change of an element would 
be transmutation, which is a more fundamental and 
deep-seated change than chemical change or any known 
kind of material change, and until the disco ver\' of 
radioactivity such changes certainly had never been 
observed. If the energ\* of radium comes from within. 
radium must be suffering a spontaneous kind of transmu- 
tation into other elements. So that, if we would avoid 
the necessity of beheving in the process of transmuta- 
tion, not as a vague possibiht}'. for example, in the sun 
and stars, under some unattainable transcendental 
condition, but as actually going on imperturbably 
around us, which the first alternative demands, we must 
seek a way of escape on the second alternative which 
requires none of these bewUdering heresies, but simply 
transfers the mystery from the radium to the great 
external unknown, and leaves it there in good company 
"\\ith many of a similar kind. 

The Ixteexal E>rERGY of Matter. 

At tliis stage it is well to ask the question. Is there any- 
thing opposed either to reason or to probability in the 
view that the energy evolved from radium is actually 
derived from an existing previously unsuspected internal 
store within the atom, and that in this process the 
element suffers a transformation into other elements ? 
How is it that such enormous stores of energy* in matter 
have remained so long unknown ? 

One of the most elusive features of energy is that you 
cannot say by mere observ'ation. or by the use of any 
instrument, how much or how httle is stored up in any 
kind of matter. For example, this flask contains a 
large quantity of an oUy yellow liquid. We cannot tell by 


simple inspection the amount of energy stored up in this 
fluid. It may be some quiet and harmless oil, which can 
be shaken vigorously with impunity, or it may be nitro- 
glycerine, one of the most dangerous and powerful ex- 
plosives. Something more than observation is necessary 
to tell us the amount of energy that may be stored within 
this substance, possibly only awaiting a slight shock to 
be evolved. The only way to find out is to try to 
explode it as thoroughly as we can, and then if it will 
not explode we may conclude that, as far as we know, it 
has no latent store of energy waiting to be loosed from 

Explosion is merelj'' a very rapid and violent type of 
chemical change, and the same general idea holds good 
for all the changes it is possible for matter to undergo. 
AVe may determine the energy evolved or absorbed in 
any change, that is, in the passage from one kind of 
matter to another kind. We have no means of telling 
the absolute amount of energy in any kind of matter. 
But the one thing of which the chemist is positive is that 
in all the material changes matter undergoes — radio- 
activity being excepted— the elements do not change 
into one another, but remain in their various compounds 
essentially unaltered. If transmutation were possible, 
and one element could be changed into another, it would 
be easy to measure the difference in the amount of energy 
of the two elements. 

As it is, the internal energy of the elements remains 
always unaffected by pre\aously known material changes, 
and therefore till recently quite unknowable. 

Before we can find out how much or how little 
energy is internally associated with the atoms we 
must be able to study a case of transmutation. The 
great stability of all elements under all conditions — 
even in the sun the identical elements which we 
know here persist, if we can rely on the evidence 
of the spectroscope — is well in accord with the view 
that all the elements contain a very large store of 
internal energy, which is never released in ordinary 


changes, but which makes them indifferent to changes 
in their environment. Thus the internal kinetic energy 
of a torpedo containing a revolving gyrostat makes 
it successfully resist deflection from its course by the 
wind and waves. The internal energy of the solar 
system, taken as a whole, is the sole reason why it 
continues to exist as a system and does not drift 

So far then from there being anything opposed to 
reason or probability in the \'iew that the atom of the 
element contains a great and hitherto unlaiown store 
of internal energy, we see that if it possessed such a store 
we could not know of it until it changed, while the greater 
the store the more would it resist change from without, 
and therefore the less likely should we be to suspect its 
existence. From this point forward we shall find that 
the more the apparent objections to the first alternative 
of internal energy are faced the less serious they appear, 
while with the second alternative of external energy the 
contrary is the case. 

Radium a Changing Element. 

Having with these preliminaries somewhat cleared the 
ground, I now msh to attempt to explain a series of 
experimental investigations which have thrown a flood 
of light upon the nature of radioactivity. Though by a 
superficial or merely external observation of radium, 
even over the period of a whole lifetime, it would hardly 
be possible to detect the least change of any kind in the 
matter itself or any exhaustion of its output of energy, 
these investigations have proved that radium, and every 
element that is radioactive, is actually changing in a very 
peculiar and definite way. These new changes in radio- 
activity are always excessively minute as regards the 
actual quantities of matter undergoing change in any 
period of time. Except in very special circumstances 
they are quite beyond the range of the most delicate 
methods of investigation previously known to the 


chemist. The methods employed in their investigation 
are in the first place wholly novel, but they are none the 
less trustworthy or definite on that account. 

Disintegration in Cascade. 

They depend on the important fact that when a 
radioactive element changes it does not as a rule do so 
once only, producing in a single step the final product of 
its change. Usually there are several successive changes 
following one another, so to speak, in cascade. Just as 
a waterfall, instead of taking one plunge into a lake, 
may cascade in a series of successive leaps from pool to 
pool on the way down, so a radioactive element like 
radium passes in its change through a long series of inter- 
mediate bodies, each produced from the one preceding 
and producing the one following. Whereas, however, 
the first change is and must be slow, the subsequent 
changes may be, and usually are, relatively far more 
rapid. But for the existence of these ephemeral, 
rapidly changing, intermediate substances, continually 
being produced and as continually changing, it is safe 
to say the mystery of radium would to-day be still 

Picture to yourselves exactly what this problem in- 
volves. Out of a remote, and so far as we know un- 
limited, past this world has gradually come into the state 
we find it to-day, and what we find is that there is a 
process knowli as radioactivity still spontaneously going 
on in matter in its natural state as it is dug out of the 
earth, which we cannot in any way stop or retard, and 
which we recognise as the intrinsic property of certain 
chemical elements. We must conclude, until we have 
evidence to the contrary, that radioactivity is not a 
process which has started recently, or that it is confined 
to the particular epoch of the earth's history we are now 
living in. So long as the radioactive elements have 
existed this process must have been going on, and, if we 
are forced to the conclusion that the radioactive elements 


are changing, is it not obvious that the changes must be 
excessively slow for any of the radioactive elements to 
have survived ? What could the methods of chemistry 
avail in such a search ? Delicate as these are to-day, 
beyond the hmit of what was even conceivable a hundred 
years ago, infinitely finer and more sensitive methods are 

The geologists tell us, and we shall find in radioactivity 
only confirmation, that the earth has existed in much the 
same physical condition as it exists to-day for hundreds 
if not thousands of millions of years. A chemist could 
probably in many cases detect the change of one 
thousandth part of one element into another, whereas we 
shall come to see that for even such a small fraction of a 
primary radioactive element to change a period of the 
order of a million years would almost certainly be 

You all know the stride that chemistry took forwards 
when it impressed into its service the spectroscope, and 
was able to detect with certainty quantities of new 
elements absolutely imperceptible in any other way. For 
example, Bunsen and Kirchoff detected by the spectro- 
scope the unknown element ccesium in the natural waters 
of the Durkheim spring in the Palatinate, but to obtain 
enough caesium for their chemical investigations they had 
to boil down forty tons of this water. Coming nearer the 
present day, Mme. Curie made an equal or even greater 
step forward when she impressed into the service of 
chemistry the property of radioactivity and discovered 
the new element radium in pitchblende, though a ton of 
pitchblende contains only two grains of radium. But 
we must improve even on this. We have to detect 
the change in a minute amount of radium which is 
changing so slowly that it appears not to be changing at 
all. The actual amount of new matter which this half- 
grain of radium bromide would produce by its change in, 
say, a month or a year, is a quantity so small that 
one has only to attempt to conceive it to be ready to 
give up the search in despair. Yet in a moment I hope 


to show it to everyone in this large room, and to demon- 
strate to you a few of its most striking properties in the 
clearest way. 

Were radium to change in one single step into, say, 
lead, which we believe to be the ultimate product in the 
main line of descent, this would be impossible. Those 
of you in the back could hardly see a quantity of lead 
equal in quantity to the whole of this radium. How 
much less then could you hope to be shown the infini- 
tesimal fraction of this small quantity which is pro- 
duced in a month or a year ? No chemist has yet 
detected lead as the final product of radium, and our 
evidence on this point is at present only indirect, but it 
is now quite conclusive. But radium does not change 
all at once in one step. At least eight intermediate 
bodies intervene, each one of which is formed from the 
one preceding it with an outburst of energy, and changes 
into the next with another outburst of energy. 

The Successive Outbursts of Energy. 

A soldier on a battlefield knows without any doubt 
when he is being fired at, but it would take him a long 
and patient examination to find out, and it would be a 
matter of only secondary interest, whether the bullets 
are made, say, of lead or of nickel. The energy pos- 
sessed by the flying bullets are their, to him, practically 
important feature. After the energy is all spent the 
bullet ceases to make its presence felt. So it is with 
radium. The energy possessed by the changing inter- 
mediate substances and evolved from them is the sole 
but sufficient evidence of their existence. After the 
energy is all spent and the change is complete, only a 
most minute and patient examination, which has still 
to be made complete, will reveal the chemical nature 
of the minute amount of dead products formed. But 
before this stage is reached, in the long succession of 
energy outbursts which accompany the change of one 
intermediate form into the next, we have a succession of 


most remarkable and obvious phenomena which enable 
us to detect the separate changes and to discover the 
whole nature and the periods of average life of all the 
intermediate bodies, although these all exist only in 
absolutely infinitesimal quantity, and not one of them 
is known, or probably ever can become known, to the 
chemist in the ordinary way. It is one of the most 
wonderful triumphs in the whole history of physical 
science that such changes should have ever been detected. 
Let us turn to the main evidence on which the view that 
radium is changing was first based. 

The Radium Emanation. 

If this specimen of radium bromide was dissolved in 
water and the liquid evaporated down to dryness in 
order to get back the solid compound, it would be found 
that as the result of this very simple operation the 
radium had lost the greater part of its radioactivity in 
the process. The penetrating 13- and 7-rays would have 
completely disappeared, and the non-penetrating a-rays 
would only be one quarter as powerful as initially. 
Then a strange thing would happen. Left to itself the 
radium would spontaneously recover its lost activity, 
little by little from day to day, and at the end of a month 
it would be not appreciably less active than it at first 
was, or as it now is. 

This appears to be in direct conflict with the state- 
ment previously made that the radioactivity of radium 
cannot be affected by any known process, but it is only 
apparently so. If we study the process carefully we 
shall find that when the radium is dissolved in water 
" something " escapes into the air, and this " some- 
thing " is intensely radioactive. It diffuses about in the 
air, but remains contained within a closed vessel, if it is 
gas-tight. In short, this " something " is a new gas 
possessing the property of radioactivity to a very intense 

We owe the greater part of our knowledge of this new 


radioactive gas to Sir Ernest Rutherford, who has given to 
it a special name. He called it the emanation of radium, 
or, for short, simply the emanation. The vague term 
" emanation " is, with our present exact knowledge of 
its real nature, apt to mislead. Some, unfortunately, 
have used the term " emanation " or " emanations " 
in speaking of the various radiations which radium emits, 
and which we have already considered in some detail. 
Sir William Ramsay has proposed the name " Niton " 
for this new gas, in order to emphasise its relationship 
to the other argon gases. However, as similar new 
gases or emanations are given by two other of the 
radioactive elements, thorium and actinium, the 
original term has been generally retained. The term 
" emanation," qualified when necessary by the name of 
the radioactive element producing it, denotes one of 
these new gaseous bodies, and it is necessary not to 
confuse this particular use with its older and more 
general uses. 

Experiments with the Emanation. 

In the laboratory, half a mile from this lecture room, 
I have a further quantity of about half a grain of pure 
radium bromide which has been dissolved in water. 
The solution is kept in a closed vessel. This morning 
I extracted the emanation from the vessel, and I have 
brought it here to show you. The radium from which 
it was derived is not in the room, it is still in the labora- 
tory half a mile away. The emanation is contained, 
mixed with air, in a little glass tube (Fig. 20) provided 
with taps for its admission and extraction, and inside 
this tube are some fragments of the mineral willemite, 
a sihcate of zinc. This mineral has the appearance of 
an ordinary cold greenish-grey stone, quite undis- 
tinguished and not very different from many of the 
common pebbles of the road or seashore. It, however, 
possesses the power of fluorescing, under the action of 
X-rays and the rays from radium, with a brilliant 

Fig. 20. — Tube containing Willemite used to exhibit the Radium Emanation. 

Fig. 21. — The Same Tube photographed in the Dark by its Own 
Phosphorescent Light. 

To face page 78 


greenish light, as you may see when I bring my capsule 
containing half a grain of solid radium bromide near to 
a block of the mineral in the dark. Let us now in the 
dark examine the tube containing the emanation and 
willemite together. We find the willemite glowing 
with a most remarkable light. Even in ordinary lamp- 
light or weak daylight the glow of the willemite is clearly 
visible. Fig. 21 shows the tube (Fig. 20), which has 
been placed in front of the camera in the dark room, and, 
as you can see, the pieces of glowing willemite have 
photographed themselves by their own light. In the 
negative the walls of the glass tube, which also are 
rendered feebly fluorescent by the emanation, are 
faintly visible. The photograph proved somewhat 
difficult to obtain, as the light, consisting almost wholly 
of green and yellow, is almost non-actinic to the photo- 
graphic plate. An isochromatic plate must be employed 
and a long exposure given. Under these circumstances 
the /^- and 7-rays from the tube, as they are not refracted 
by the lens, themselves fog the plate uniformly to a con- 
siderable extent. The photograph gives no idea of the 
beauty of the original tube. Willemite glowing in the 
emanation of radium is one of the most beautiful sights 
I know, and considered with reference to the origin of 
its light and all that the phenomenon foreshadows for 
humanity, it raises feelings which only a poet adequately 
could express. 

What is the emanation of radium ? I shall treat this 
question to-night solely as though the emanation was 
a body with no connection whatever with radium, 
because a knowledge of its own nature is necessary 
before its real relation to radium can be appreciated. 
In the first place, it is intensely radioactive on its own 
account — that is to say, it gives out the new kinds of 
rays very similar in character to those given by other 
radioactive bodies and capable of producing the same 
effects. What I am about to say refers only to a tube 
in which the radium emanation has been confined for 
some hours. At first the emanation gives only a- but 


no ^- or 7-rays, as we shall consider more nearly later 
(Chapter IX.). 

This tube, in which the emanation is confined, glows 
in the dark because the phosphorescent willemite it 
contains is being bombarded by the rays from the 
emanation. Some of these rays penetrate the glass 
walls of the tube, as you may see if I bring the X-ray 
screen between your eyes and the tube. Moreover, if 
a very thin plate of metal is interposed at the back of the 
screen it does not perceptibly diminish the effect, for 
the rays from a tube containing the emanation, like the 
radium-rays themselves, are capable of penetrating a 
considerable thickness of metal. They consist, in fact, 
of a-, jS- and 7-rays together. Any of the other phos- 
phorescent bodies — for example, zinc sulphide — would, 
if placed inside this vessel with the emanation, glow in 
its characteristic way just as if exposed to radium itself. 
Similarly, a photographic plate would be fogged almost 
instantly, and an electrified silk tassel would be dis- 
charged at once by the rays proceeding from the emana- 
tion confined in this tube. The similarity between the 
a-rays from the emanation and those from radium itself 
have been proved by exact physical experiments. 

The Condensation of the Emanation by Cold. 

The next point is that the emanation is not a solid form 
of matter dispersed like fine particles of smoke in the air 
which carries it. It is a true gas. This has been proved 
by innumerable experiments ; but I wish to show you 
one which is particularly beautiful, and which has, I 
think, convinced everyone who has ever seen it per- 
formed that the emanation of radium is a true gas with 
the property of radioactivity. It was first performed 
by Professor Rutherford and myself in Montreal in 
November, 1902. If the emanation is a gas there ought 
to be some temperature, though, perhaps a very low 
one, at which it loses its gaseous form and is condensed 
or frozen. All our attempts to effect such a condensa- 


tion at temperatures down to —100° Centigrade had 
proved futile, and we had no means of obtaining the 
very low temperatures now daily employed in a modern 
laboratory But a liquid air machine was given to the 
laboratory by its generous founder, and on its first run 
the emanation of radium was successfully condensed. 
Exact experiments showed that the emanation is 
condensed quite sharply when the temperature falls 
below -150° Centigrade (or -238° Fahrenheit), and it 
volatilises and again resumes its gaseous state quite 


Fig. 22. 

sharply when the temperature rises above this. We 
shall perform the experiment in the following manner 
(Fig. 22). To one of the tubes of the vessel containing 
the emanation is attached a rubber blowing-ball, for 
blowing out the emanation. The other tube is connected 
with a U-tube of glass containing some fragments of 
willemite, immersed in a vessel of liquid air and so kept 
at the very low temperature of about - 183° Centigrade 
or - 300° Fahrenheit, into which the emanation is 
blown. Exposed to this extreme cold the emanation 


instantly loses its gaseous state and condenses in the 
tube. To make the experiment more striking, between 
the tube containing the emanation and the cooled U- 
tube I have interposed several yards of narrow tubing 
which the emanation has to traverse before reaching 
the tube in which it condenses. As you see, when I 
open the taps and gently blow a blast of air to sweep 
out the emanation into the cold U-tube, the willemite 
in the cold tube suddenly shines out brilliantly, at the 
point where the emanation condenses. 

So long as the U-tube is kept in the liquid air the 
emanation will remain there, though I continue to send 
a gentle blast of air from the bellows. But a few 
moments after taking the tube out of the liquid air, it 
warms up to the point ( - 150° Centigrade) at which 
the emanation again resumes its gaseous form, and now 
we can blow it out with a single puff of air. See ! I 
blow it out through the narrow tubing, which I have 
connected to the (J -tube, into a large flask dusted over 
its inside surface with the phosphorescent sulphide of 
zinc. In the dark the globe shines out with a soft white 
light like some fairy lantern, and I can see to read my 
watch by its light. The physiological effects of the 
radium emanation are imperfectly investigated and may 
be potent. This is a field of investigation I personally 
have no desire to explore, so that we must not forget to 
cork the globe and so prevent the emanation from dif- 
fusing out into the air of the room. 

The Infinitesimal Quantity of the Emanation. 

After this demonstration you may have some difficulty 
in really believing that the actual amount of gaseous 
emanation which has produced these beautiful effects is 
almost infinitesimal. By making use of the same pro- 
perty — its condensation by liquid air — the actual volume 
occupied by the radium emanation freed by freezing from 
all other gases was measured by Sir William Ramsay 
and myself. Imagine a bubble of air the volume of a 


good-sized pin's head, say, one cubic millimetre, or one 
fifteen-thousandth part of a cubic inch. It would re- 
quire thirty times more emanation than was actually 
employed in the last experiment to fill a bubble of this 
size. Of course, in the experiments this small quantity 
of emanation was mixed with a considerable volume 
of air for convenience in manipulation. The actual 
quantity of emanation accumulating in a radium pre- 
paration is known with accuracy to be 0-6 cubic milli- 
metre per gram of radium (element). 

It requires a distinct step for the mind to assimilate 
the important fact that the property of radioactivity, 
which so far has been studied only in solid substances 
and minerals, could be shown equally by a gas, and this 
fact accounted for the ,true nature of the emanation 
remaining largely unrecognised even after the con- 
clusive experiment I have shown you. There is, of 
course, nothing contrary to the nature of radioactivity 
in the fact that it is shown by a gas. When we apply 
Mme. Curie's theory that radioactivity is an intrinsic 
property of the atom, and of the element in question, 
the difficulty is not that the emanation is a gas, for many 
elements are gases, but how it is that a new radioactive 
element, such as the emanation undoubtedly is, should 
result when radium compounds are dissolved in water, 
and this question we have purposely deferred. 

The Radioactivity of the Emanation. 

The emanation, as we have employed it in our 
experiments, is mixed with ordinary air, and in this 
way it can be dealt with and treated like any other gas. 
We have blown it through tubes from one end of the 
lecture table to the other. If it had been an ordinary 
gas, like air, no one could have seen it, or known what 
became of it. But being intensely radioactive, although 
its actual quantity is almost inconceivably small, the 
radioactivity serves as a sufficient evidence of its 
presence or absence, making it, as a matter of fact, far 


easier to work with and to investigate than an ordinary 
gas in ordinary quantity. If a mining engineer wished 
to know how the air he pumped into his mine got dis- 
tributed among the various shafts and pits, he could 
not do better than to put a little radium emanation into 
the entering air, and then subsequently to take samples 
at various parts of the mine, and have them tested for 
content of radium emanation by a gold-leaf electroscope. 
Many other practical problems in the flow of gases, 
which are difficult to solve by ordinary methods, might 
be readily solved by the help of this new gas. 

The Chemical Character of the Emanation. 

It has even been found possible to settle the chemical 
nature of this new gas, and to place it in its proper 
family of elements in the periodic table. Almost all 
gases, according to their various natures, are absorbed 
when subjected to the action of various chemical re- 
agents. Thus oxygen is absorbed by phosphorus, 
hydrogen by heated copper oxide, nitrogen by heated 
magnesium, and so on. The exceptions, namely, gases 
which are not absorbed by any reagents and which will 
not combine with anything, are the newly discovered 
gases of Lord Rayleigh and Sir William Ramsay — argon, 
helium, neon, etc. — which exist in atmospheric air. 
The quantity in the air of these gases is extremely 
minute except in the single case of argon, which is 
present to the extent of one per cent. The radium 
emanation, like argon, is not absorbed by any kno\vn 
reagent, and does not appear to possess any power of 
chemical combination. It may be passed unchanged 
through absorbents, or subjected to drastic chemical 
treatment which would suffice to absorb every known 
gas except those of the argon type, and the conclusion 
has been arrived at that the emanation is an element 
of the same family nature as the argon gases. Like 
them, it exists in the form of single atoms — ^that is, 
its molecule is monatomic. Radium, on the other hand, 


in its chemical nature is extremely similar to barium, 
strontium, and calcium, a family known as the alkaline- 
earth elements. None other of the argon elements 
or the alkaline- earth elements are radioactive, and yet 
the radioactive elements are quite normal in their 
chemical properties, closely resembling ordinary ele- 
ments, and being associated in the clearest and closest 
way with one or other of the old well-known types or 
families. More recently, by using quantities of radium 
about fifteen times as great as those used to-night in 
our experiments, it has been possible to obtain enough 
of the emanation for it to be possible to photograph its 
spectrum. This proves to be a new and characteristic 
bright-line spectrum, resembling in general character the 
spectra of the other argon, gases, but absolutely distinct. 
It has been found possible to obtain some idea of 
the density of the emanation of radium, and therefore 
of the weight of its atom, from experiments on the rate 
of its diffusion from one place to another. These 
indicate that the gas is extremely dense — denser pro- 
bably than mercury vapour — and therefore that it 
has a very heavy atom. Finally, by means of a new 
special micro-balance thousands of times more sensitive 
than the most delicately constructed chemist's balance, 
the emanation has actually been weighed by Sir William 
Ramsay and Mr. Whytlaw-Gray. These experiments 
and the whole of the available evidence agree in indicat- 
ing that the atomic weight of the emanation is 222, 
which is four units below that of radium, and there- 
fore is the fourth heaviest known. 

The Heat evolved by the Emanation. 

The heat given out by a gram of radium, as we 
have seen, is 133 calories per hour, but it must be 
understood that this refers to radium in its normal 
condition containing its full quota of emanation. After 
solution in water, that is, after the emanation is ex- 
tracted, the radium gives out heat to the extent of only 


thirty-three calories per hour, while the emanation 
produces one hundred calories per hour. That is to 
say, the emanation of radium gives three times as much 
energy as the radium from which it is derived, although 
the actual amount of matter in the emanation is itself 
practically imperceptible. 

Now, perhaps it is easy to understand how it is that 
the minuteness of the quantities of material offers no 
barrier in the investigation of radioactivity. Mass 
is not the only consideration. A very small bullet 
suffices to work terrible havoc, in spite of its smallness, 
by means of the kinetic energy with which it is impelled. 
A little torpedo, stuffed full of imprisoned energy in 
the form of explosives, suffices to sink an enormous 
battleship. A quantity of emanation, which certainly 
does not weigh a hundred-thousandth part of a grain, 
gives out enough energy to produce effects plainly visible 
to you all at the very back of the room. 

If, instead of the thirtieth part of a pin's head full, 
we could obtain a pint of this gas — and to obtain such a 
quantity half a ton of pure radium would be required 
— it would radiate the energy of a hundred powerful 
arc-lamps. Indeed, as Rutherford has said, no vessel 
would hold it. Such a quantity would instantly melt 
and dispel in vapour any material known. 

The Decay of the Emanation. 

These new facts, which transpire the moment we 
begin to make a systematic investigation of the radio- 
activity of radium, make the second alternative, that 
the energy of radium is derived from outside, well-nigh 
incredible. For to account for the energy evolved from 
the emanation we must suppose all space to be every- 
where traversed by new and mysterious forms of radiant 
energy of such tremendous and incredible power that 
the explanation is harder to believe than the fact it 
is supposed to explain. To avoid the necessity of sup- 
posing that the energy resides within the comparatively 


small amounts of radioactive matter in existence, we 
must fill the whole of external space with radiant 
energy of a similar order of magnitude. This is strain- 
ing at a gnat and swallowing a camel. 

Fortunately there is a crucial test by which we are 
now in a position to decide between the two alternative 
views. Let us apply the theorem we have already 
deduced (p. 70) from general principles. If the energy 
comes from within the radioactive matter, its radio- 
activity must in course of time diminish and decay — 
the more rapidly the more powerfully radioactive it is. 
Whereas, if the energy comes from the outside, however 
powerful the radioactivity may be, there is no reason 
why it should not continue indefinitely with undiminished 

We have seen that the emanation is, mass for mass, 
far more intensely radioactive even than radium, and, 
if the energy comes from within, it is to be expected that 
the activity of the emanation will be short-lived in 
comparison with that of radium, whereas, if the energy 
is derived from outside, no such decay is to be antici- 
pated. Does the radioactivity of the radium emanation 
diminish or decay, or does it continue permanently ? 

The answer to this question is that the radioactivity 
of the emanation rapidly decays away from day to day. 
Four days hence the activity will be but one-half of 
what it now is. In eight days the activity will be re- 
duced to one-fourth, in twelve days to one-eighth, in 
sixteen days to one-sixteenth, and so on, diminishing 
practically to zero at the end of a month in a descending 
geometrical progression with the lapse of time. 

The light from the glowing willemite in this tube, 
when it is left entirely to itself, will gradually fade, and 
at the end of a month will have died almost completely. 
Vast as is the store of energy in matter which is released 
in the radioactive process, it is not infinite, and in the 
radium emanation we have an example of a change 
proceeding so rapidly that only a few weeks are necessary 
for its completion. 


The Reproduction of the Emanation by 

Half a mystery is usually greater than the whole, and 
in science when mysteries begin to appear on all sides, 
the explanation is often near at hand. We dissolved 
a compound of radium in water, and the greater part of 
its activity disappeared in the process. Then little 
by little the lost activity was spontaneously recovered, 
and at the end of a month the radium was not appre- 
ciably less active than at first. The disappearance of 
the greater part of the activity after solution was ex- 
plained by the fact that an extremely radioactive gas 
— the emanation — was liberated during the act of 
solution, and this carried away with it the whole of the 
radioactivity which the radium had lost. But, lo ! 
while the radium slowly recovered its original radio- 
activity, the emanation lost what it had at first pos- 
sessed. A quantitative examination of these two pro- 
cesses of decay and recovery at once showed that the 
total radioactivity had not been affected, but had 
remained constant in spite of the treatment to which 
the radium had been subjected. This is a fundamental 
law of universal application to all radioactive bodies, 
and it has been called the Law of the Conservation of 
Radioactivity. Whatever you do to any radioactive 
substance you cannot artificially alter the total radio- 
activity, though you may frequently, as in this example, 
divide it into several parts, for reasons that will soon 
be clear. 

It is easy enough on the first alternative to account 
for the comparatively rapid decay of the activity of 
the emanation of radium. It is dissipating its internal 
store of energy so rapidly that it is soon exhausted. 
It is a clear case of a short life and a merry one. But 
how is the gradual recovery of the radioactivity of the 
radium in the course of time to be explaii^ed ? This 
is the key to the whole problem, and on the second 


alternative no answer whatever can be given. The 
explanation that the energy of radioactive substances 
is derived from outside is not merely incredible. It is 
altogether insufficient. 

Imagine that a month has elapsed, and that the 
radium, which has now recovered completely its lost 
activity, is again dissolved in water and evaporated 
down to dryness exactly as before. Again you would 
find that in the process the radium had lost the same 
large proportion of its radioactivity, and again you would 
obtain from it a new amount of emanation no less than 
that which is on the table to-night. Repeat the experi- 
ment as often as you please and you will find the result 
always the same. While the emanation you separate 
from the radium is decaying away from day to day, 
a fresh crop is being spontaneously manufactured by the 
radium. The change of the radium into the emanation 
is, as a matter of fact, only the first of a long series of 
successive changes of a similar character. The gaseous 
emanation in turn rapidly changes into a third body, 
not a gas, called radium A; this into a fourth, called 
radium B; and so on. Nine successive changes are at 
present known, which we shall have to give some account 
of later. 

Atomic Disintegration. 

This explanation of radioactivity, which has come 
to be known as the theory of atomic disintegration, 
was put forward by Professor Rutherford and myself 
as the result of a long series of experimental investi- 
gations carried out in the Macdonald Physical and 
Chemical Laboratories at McGill University, Montreal. 
It has, since, not only shown itself capable of interpreting 
all the very complicated known facts of radioactivity, 
but also of predicting and accounting for many new ones. 
Although on the surface a revolutionary addition to the 
theories of physical science, it must be remembered that 
it is the facts of radioactivity which are really revolu- 
tionary. While accommodating these strange new facts 


the disintegration theory conserves in a truly remark- 
able way the older established principles of physical 
science. Without such a guiding hypothesis, recon- 
ciling the old and the new, it is safe to say that the facts 
of radioactivity would ultimately have wrought a far 
greater change in scientific theory than has actually 
taken place. Although the emanation of radium is 
not and, as we shall come to see, never can be obtained 
in palpable quantities — it is changing too rapidly for 
that — we know almost as much about its nature and 
properties as we do about any of the older gases. 

Radioactive Equilibrium. 

A very important point is that just as we cannot 
really alter the radioactivity of a body artificially in 
any way, we cannot and do not in any process influence 
the rate at which the emanation is being formed from 
radium or the rate at which it in turn spontaneously 
changes. The same amount is always in existence 
whether you separate it or not. The apparent constancy 
of the radioactivity of radium is not the real constancy 
to be expected of a transforming mechanism. It is 
the apparent constancy produced by the equilibrium 
between continuous and opposing changes, on the one 
hand the rapid decay of the part of the radioactivity 
due to the emanation, and on the other the regeneration 
of fresh emanation as fast as the old disappears. This 
process of regeneration is always going on at a perfectly 
definite and unalterable rate, and the property of pro- 
ducing a certain definite amount of emanation in a given 
time is as mu( h a part and parcel of the very nature of 
radium — and indeed the best and most easily applied 
qualitative and quantitative test for the presence of 
radium in the minutest quantity that we possess — as 
is its power of giving the rays which lit up the X-ray 
screen and discharged the silk tassel, or as its power of 
generating heat. 


Energy of Radioactive Change. 

All of these properties are but the various aspects 
of a single primary cause. The element radium is 
changing, so slowly it is true, that at first sight it appears 
not to be changing at all, and yet with so tremendous 
and unparalleled an evolution of energy that the trans- 
formation of an otherwise imperceptible part of its 
mass is accompanied by an amount of energy so great 
that the change could not by any possibility have 
remained unknown. The emanation is the first main 
product of the change of radium. If the emanation 
were like lead or any ordinary element it would take 
years of accumulation and the most minute and patient 
investigation to detect it's production. But it is not. 
The emanation changes again into a third type of matter 
we have not yet considered (the nature of which does 
not yet concern us), but whereas it would take hundreds 
of years for any appreciable fraction of the radium itself 
to change, the change of the emanation is rapid and 
goes to practical completion within a single month. 
It is precisely on this account that we can work with 
and detect such almost infinitesimal quantities. What 
may be termed the material evidence of radioactive 
change, the detection, by purely chemical or spectro- 
scopic methods, of the materials formed in the changes, 
is still scanty, although not altogether lacking. But 
the radioactive evidence, which depends not on the 
material produced, but upon the energy evolved, and 
on the way in which the energy is manifested, is abundant 
and sufficient. So long as the energy evolved is suffi- 
cient in quantity, and of a kind suitable for detection 
in any of the various ways I have illustrated, the actual 
quantity of matter producing the energy is of no 


All Radioactive Changes equally Detectable. 

But the amount of energy produced by any change 
depends not only on the quantity of matter changing, 
but also on the time the change lasts, that is, on the 
period of life of the changing matter. Chemical and 
spectroscopic methods of detecting matter depend on 
quantity, whereas radioactive methods depend on 
quantity divided by life. The shorter the life of the 
changing substance the less of it is necessary for its 
detection by means of radioactivity. This is a merely 
preliminary and tentative indication of the operation 
of an exactly compensating principle of great importance, 
which later it will be possible to formulate as a general 
law. Its result in the long run is this. Each of the 
ephemeral intermediate substances in the cascade of 
changes comes equally within our powers of investigation, 
whether it changes slowly or rapidly, whether it lasts 
long enough to accumulate in ponderable quantity, or 
whether it is changing so rapidly that it 

Like snow upon the desert's dusty face. 
Lighting a little hour or two, is gone. 



The Connection of the a-P article with Radio- 
active Changes. 

Last week we studied the first step in the evidence 
that radium is changing, and considered in some detail 
the chief practical reason why such changes have proved 
within our powers of discovery, namely, that the change 
is not single but proceeds in cascade from stage to stage, 
producing ephemeral intermediate transition-forms, of 
which the radium emanation is one, almost inconceiv- 
ably minute in their actual quantity but evolving in 
their next change very large amounts of energy, by 
means of which it is possible to trace them and study 
their nature with ease. We considered the first product 
of the change of radium, namely, the emanation of 
radium, its nature and properties, and its continual 
production from radium. We reserved purposely the 
examination of the connection between radium and the 
emanation it produces. Now I wish to combine with 
the knowledge we have gained of the nature of the radium 
emanation that already considered (Chapters III. and 
XV.) with reference to the nature of the a-particle. 

A radium salt is dissolved in water, and the im- 
prisoned emanation, which was formed but stored 
during the previous month throughout the whole mass 
of the substance, is thereby liberated and escapes. 
The radium left to itself continues to produce fresh 
emanation at a steady rate. The released stores of 
emanation begin to lose their radioactivity. We shall 

93 8 


confine our attention at first solely to the case of the 

When radium in this way is freed from all previously 

formed emanation it still gives out a-particles, although 

only now one-fourth as many as it gives out when it 

contains its full quota of emanation and other products. 

^ These a-particles we regard as pro- 

O^^ >«-v duced from the radium atom in the 
} "~^ ( ) same change as that in which the 
emanation is produced. The emana- 
Radium. Emanation. ^. -^ regarded, in fact, as radium 
Fig. 23, , , f .-1 

that has lost one a-particle. 

This, which is a perfectly general point of view, was 
proved from the first by the consideration of a mass of 
evidence accumulated with reference to the similar 
changes going on in the element thorium, but much of 
this may be left for later treatment. The evidence 
that has since been accumulated enables the same deduc- 
tion to be more simply made, and this alone need be 
considered. Henceforth the original reasoning as to 
the nature of atomic disintegration, although it was, 
when first put forward, very complete and convincing 
to those acquainted with the whole of the experimental 
facts, will be largely replaced by the more direct evidence 
since obtained. 

Helium and the a-PARTicLE. 

We have seen in considering the nature of the a-rays 
that they are now regarded as due to the flight of swarms 
of helium atoms expelled from the radioactive substance 
with an almost inconceivable speed of from 8,000 to 
12,000 miles per second. Long before the real nature 
of the a-particle was known, helium had been first pre- 
dicted to be and then proved experimentally to be a 
product of the radioactive changes of radium, and this 
chapter in the development of the subject has something 
more than an historical interest. 

Before proceeding, one underlying consideration 


governing the view that an atom of helium and an atom 
of emanation are simultaneously formed when an atom 
of radium disintegrates, must be made clear. It refers 
to the relative quantities of each product, helium and 
emanation, which it may be expected will be formed 
by the continuous operation of the process. Helium 
we know is not radioactive, and therefore there is no 
evidence that helium is changing in any way, and we 
may in this sense refer to it as one of the ultimate pro- 
ducts of the change. The emanation, on the other hand, 
is changing so rapidly that the change may be regarded 
as complete in the course of a single month. The bodies 
it is changing into we have not yet dealt with, and they 
do not immediately concern us. 

Now a changing substance, like the emanation, 
cannot possibly accumulate in quantity with lapse of 
time beyond a certain very small extent. It is true 
it is constantly being formed from radium in the same 
way as helium, but whereas the helium, being a stable 
substance, may be expected to accumulate in a quantity 
that is proportional to the time that elapses, the quantity 
of emanation will not increase beyond a certain point. 
For in a very short time after the process of accumulation 
of emanation from the radium begins, as much emana- 
tion will itself change as is formed, and the quantity 
from that time on will remain constant. This condition 
is known generally as " radioactive equilibrium," and 
when we speak of the emanation being in equilibrium 
with the radium we mean that the quantity of emana- 
tion has reached a maximum and does not further 
appreciably increase with lapse of time. In the case of 
the emanation practical equilibrium results in the com- 
paratively short time of a few weeks. That is to say, 
however long radium is left undisturbed to accumulate 
its emanation, the quantity of the latter never exceeds 
a practically almost infinitesimal one, for it is a quantity 
which is produced from the change of the radium in 
quite a short period of time. Its quantity is therefore 
excessively minute. It is so very minute that were 


it not changing and evolving energy it would not be 
detectable by any ordinary method. 

You will see that it follows at once from this point of 
view that if any element were produced in the dis- 
integration of radium, which itself did not change but 
was permanent, then on the one hand, owing to the 
extreme smallness of the amount formed, it would not 
be easy in a short period to obtain evidence of its pro- 
duction, by means of ordinary chemical tests, but, on 
the other hand, the quantity would go on accumulating 
indefinitely with lapse of time. 

The Ultimate Products. 

As we saw last week, the first evidence of atomic 
disintegration was dynamical and due solely to the 
energy which is evolved in the process. The answer 
to the question as to what are the ultimate products of 
atomic disintegration must be looked for on quite 
different lines. The ultimate products formed will be too 
small for detection in the ordinary way by the statical 
methods of chemistiy and physics, but they will accumu- 
late indefinitely. 

Since the processes go on steadily, so far as we know, 
in the minerals in which the radioactive elements are 
found, the ultimate products, formed through past ages 
of disintegration, must accumulate therein from one 
geological epoch to the next. So that at the present 
day one ought to find in the radioactive minerals the 
ultimate products of the disintegration process, accu- 
mulated in sufficient quantity to be capable of detection 
by the ordinary methods of chemistry. 

Now the radioactive minerals are always very com- 
plex, and contain a very large proportion of the total 
number of elements known, so that in most cases it is 
impossible to deduce very much from this evidence. 
Nevertheless, there was one clear definite exception, and 
that was the element helium. Another definite but less 
unequivocal exception was the element lead. 


Discovery of Helium, Solar and Terrestrial. 

The history of our knowledge of helium is unsurpassed 
by that of any other in interest. Its very name (from 
rjX,Lo<i, the sun) stands witness to the fact that it was 
known to exist in the sun as an element before it was 
known to exist on the earth at all. It was discovered 
in 1868 by the spectroscope in the sun's chromosphere, 
by means of the characteristic bright yellow line in its 
spectrum, which is technically known as " D3". Then, 
in 1895, Sir William Ramsay discovered it in certain 
minerals found in the earth's crust, and made a syste- 
matic investigation of its physical and chemical nature. 
It is a gas, the second lightest known, only twice as 
dense as hydrogen, and for long was the only gas which 
successfully resisted all efforts made to liquefy it by 
extreme cold and pressure. In 1908, however, Kammer- 
lingh Onnes succeeded by the exercise of wonderful 
experimental skill and persistence in reducing helium 
to the liquid state, attaining thereby a far lower tem- 
perature (270° Centigrade, or only 3° from the absolute 
zero of temperature) than has ever before been reached. 
It is readily evolved from the minerals in which it is 
found, either by heating them or by dissolving them, 
but once evolved it cannot again be absorbed by the 
minerals or by any other substance known. Indeed, 
helium resembles argon perfectly in chemical nature, 
in that it is quite without any combining power, and 
exists free as single atoms without being known to form 
compounds of any kind whatever. Its atomic weight 
is four (hydrogen=l). Sir William Ramsay drew 
attention to the fact that all the minerals in which he 
found helium contained either uranium or thorium. 
This was before the days of radioactivity, and for long 
the origin of the helium — a non-condensable, non- 
combining gas — in minerals containing uranium and 
thorium was a matter for comment and speculation. In 


certain cases the volume of helium evolved is nearly 
a hundred times as great as the volume of the mineral 
in which it is contained. 

Prediction of the Production of Helium. 

The disintegration theory enabled Professor Ruther- 
ford and myself at once to give a probable explanation 
which has since proved to be correct. We regarded 
helium as one of the ultimate products of the disintegra- 
tion of the radioactive elements, radium, uranium, and 
thorium. Forming during the long ages of the past 
throughout the mass of the mineral, which is often of a 
glassy nature, it is unable to escape until the mineral 
is heated or dissolved, and it steadily accumulates with 
the passage of geological time. We ventured to predict 
that helium was one of the ultimate products of radio- 
active changes, being formed in Nature from radium, 
uranium, and thorium, excessively slowly, but still 
fast enough to ensure that all minerals containing these 
elements must contain helium also. This has since been 
proved to be the case. It is true that in certain uranium 
minerals — e.g., autunite and carnotite, the amount 
present is often excessively minute, but these also are 
just the minerals which it is believed are of extremely 
recent geological formation. Indeed, the ratio between 
helium and uranium or thorium in minerals is now 
one of the recognised methods of estimating their age. 

From this point the work proceeded along two 
separate lines. Rutherford, in an exhaustive examina- 
tion of the nature of the a-rays, which we have already 
considered, proved first that they consisted of positively 
charged atoms expelled with great velocity. At first 
their mass was given as twice that of hydrogen, on the 
assumption they carried one atomic charge. Then, as 
the sequel to the beautiful counting experiments we have 
considered, it was proved in 1908 that each a-particle 
carries two atomic charges of positive electricity. 
Therefore the mass of the a-particle is four, that is to 

Fig. 24. — Original Spfxtrum-Tube in which the Formation 
OF Helium from Radium was first observed. 



from J. 


1 1 

1 _^__^„ 1 


Red 1 

1 Violet 



Fig. 25. — Dr. Giesel's Photograph of the Spectrum of the 
Gas from Radium. 

II 20 minutes', III 5 minutes' exposure. I is the Spectrum of Helium, 
IV that of Hydrogen for comparison. 

To face p. 99 


say, it is the same as that of the atom of helium. This 
made it very probable, therefore, that the a-particle 
is an atom of helium. 

Production of Helium from Radium. 

The prediction that helium was a product of radio- 
active changes was proved directly by Sir William 
Ramsay and myself in 1903. We chose for the parti- 
cular case of radioactive change studied that of the 
emanation of radium, since it is rapid, and the emana- 
tion can readily be obtained, free from other gases, 
first by the action of suitable absorbents, and finally 
by condensing it with liquid air and removing the gases 
not condensed with a pump. So purified, it was sealed 
up in a small spectrum tube, so that the spectrum of 
the gas could be examined at will, and then it was left to 
itself. At first no helium was present. Helium, not 
being condensable by liquid air, could not have been 
present in the tube as first prepared. But in the course 
of three or four days, as the emanation disintegrated, 
the spectrum of helium gradually made its appearance, 
and finally the whole characteristic spectrum of helium 
was given by the tube. Fig. 24 shows a photograph 
of one of the original spectrum tubes in which the pro- 
duction of helium from radium was proved. This 
observation of the production of the element helium from 
the radium emanation, and therefore (since the emana- 
tion in turn is produced from radium) from the element 
radium, has since been verified and confirmed by 
numerous investigators in various parts of the world. 
It has also been found by Debierne in a similar manner 
by the spectroscope that actinium, a radioactive sub- 
stance found by him in pitchblende, produces helium. 
Dr. Giesel has actually succeeded in photographing the 
spectrum of the gases generated by radium, and one of 
his photographs is reproduced in Fig. 25. It represents 
four separate spectra, one below the other in parallel 
strips. The uppermost (I) is ordinary helium. The 


second and third (II and III) are two photographs 
obtained from the gas generated by radium. In the 
second an exposure of twenty minutes, and in the third 
one of five minutes were given. The lowest spectrum 
(IV) is that of hydrogen. It will be seen that many 
of the helium lines are present in the spectrum of 
the gas from radium. The other lines are those of 
hydrogen, due, no doubt, to the presence of a trace of 
moisture. The figures above and below the plate refer 
to the stronger lines of helium and hydrogen respec- 
tively clearly visible in^ photograph II. They refer to 
the wave-lengths in Angstrom units (10 "^"^ metre). 
It must be remembered that the (visually) brilliant 
yellow line D3, owing to its colour, appears far less 
intense in the photograph than the blue and violet lines. 

Production of Helium from Uranium and 

I was engaged for four years in an attempt to detect the 
production of helium from the primary radio-elements 
uranium and thorium, and succeeded in proving in 
both cases that helium is produced, and, moreover, 
that the rate of production is almost exactly what is to 
be expected from the theory of atomic disintegration. 
This quantity is about one five-hundred-thousand- 
millionth of the mass of the uranium or thorium per 
annum ! A photograph of the apparatus employed, 
as it stood in the Physical Chemistry Laboratory, is 
shown in Fig. 26. These are seven exactly similar 
arrangements side by side, each of which is quite separate 
and unconnected with the others. Each consists 
essentially of a large flask, capable of holding a con- 
siderable quantity of the material experimented upon 
in the form of solution. Each is provided with a peculiar 
form of mercury tap, which, while it serves perfectly 
to keep out the atmosphere from the flask for an in- 
definite time, can at any moment be opened by sucking 
down the mercury in the barometer tubes, so that the 

w :i 

0. S 

To face p. loo 


accumulated gases from the flask can be extracted and 
tested for helium without admitting air. Air has been 
the great trouble. A pin's-head-full of air left in the 
whole of the large flask or in the solution, or leaking 
in during the periods of accumulation, would completely 
ruin the experiment. Most of the elaborations of the 
apparatus have to do with .the preliminary thorough 
removal of the air from the apparatus before the ex- 
periments are commenced. The methods of testing for 
helium are also entirely new. They depend on the 
power I found was possessed by the metal calcium, when 
heated to a very liigh temperature in a vacuum, of ab- 
sorbing the last traces of all gases except the gases of 
the helium and argon type. In this way the minute 
amount of helium produced (usually not more than a 
thousandth part of a cubic millimetre) is freed perfectly 
from every other trace of gas and water vapour. Finally, 
it is compressed by means of mercury into the smallest- 
sized spectrum tube that can be made and its spectrum 
examined. As shown in numerous special experiments, 
the D3 line of the helium spectrum can be detected with 
certainty if one millionth part of a cubic centimetre, or 
one five-thousand-millionth part of a gram of helium 
is present. Tliis is certainly the smallest quantity of 
any element that has ever been detected by the spectro- 

By frequently repeated experiments one can find 
for each flask a period of accumulation that must be 
allowed before helium can be detected in the expelled 
gases, and so one can obtain a measure of the rate of 
production of helium. In this way I have obtained 
helium repeatedly from both uranium and thorium 
salts, and the rate of production has been found to be 
of the same order as that previously calculated from the 
disintegration theory. For the case of uranium the 
rate of production is about two milligrams of lieiium 
from a thousand tons of uranium per year. 


Identity of the a-PARTiCLE and Helium. 

The position is then this: heUum has actually been 
found to be produced from the various radioactive 
substances — radium, thorium, uranium, actinium — 
which have in common ihe fact that they all expel 
a-particles. The mass of these particles has been 
measured and found to agree with the mass of the 
helium atom. All a-particles have been proved to have 
the same mass and to differ only in the initial velocity 
of expulsion, whether expelled from radium itself, 
from the emanation, from actinium, uranium, thorium, 
or any other of the bodies which expel them. Hence 
we are justified in concluding that the a-particle is an 
atom of helium, or at least becomes one after the 
velocity with which it is expelled is lost and it is brought 
to comparative rest. 

One further step in this long converging series of 
experiments clinches the argument. We have seen 
that the a-particle, though but feebly penetrating, has 
a very definite small penetrating power. Now glass is 
a substance that can be blown to an excessive degree of 
thinness and yet retain to the full its air-tight properties. 
I have succeeded in blowing small windows of glass thin 
enough to allow the a-particle to get through, and yet 
strong enough and tight enough to stand the pressure 
of the air on one side when there was an almost perfect 
vacuum on the other. So that it ought to be possible, 
if the a-particle is an atom of helium, by storing the 
radioactive substance in a very thin-walled air-tight 
glass vessel, to get helium produced outside the vessel, 
although no helium or other gas in the ordinary state 
confined inside the vessel could escape. This experiment 
has been performed by Rutherford and Royds with a 
large quantity of radium loaned by the Austrian Govern- 
ment. The emanation from the radium, which gives 
a-particles and has been shown to give helium, was stored 
in an excessively thin- walled but still perfectly gas-tight 


capillary tube, enclosed within a wider vessel. After 
some days the gas in the outer vessel was found to con- 
tain helium. It was proved that when helium was 
stored in the inner tube, none got through into the outer 
vessel. This final experiment clinches the proof that 
the a-particle is an atom of helium. 

The FmsT Change of Radium. 

So we are justified in writing the first disintegration 
suffered by radium: 



Radiutt). Emanation. Helium. 

Fig. 27. 

There is a great deal of evidence which proves that one 
atom of a radioactive body expels but one a-particle 
at each disintegration. Hence, since the atomic weight 
of radium is 226, and that of helium 4, the atomic weight 
of the emanation is presumably 222. This is the value 
obtained by direct experiment (Chapter V.). 

The above diagram is typical of no less than nineteen 
different radioactive changes, in all of which an atom 
of mass between 240 and 206 expels an a-particle, or 
helium atom, of mass 50 or 60 times less. By the usual 
dynamical law it is to be expected that the heavy 
residue of the original atom, whatever it is, should 
recoil in the direction opposite to that in which the 
a-particle is expelled with a velocity between 50 and 
60 times less than the a-particle, that is to say, with a 
velocity between 150 and 250 miles a second. The 
kinetic energy of this recoihng atom, since it depends 
upon the mass multiplied by the square of the velocity, 
will also be between 50 and 60 times less than that of 
the a-particle. The velocity and kinetic energy pos- 
sessed by a recoiling atom, though greatly inferior 


to that of an a-particle, are nevertheless greatly superior 
to that possessed by an ordinary gas molecule at any 
attainable temperature. 

Radioactive Recoil. 

The phenomenon of radioactive recoil comes into 
evidence in a very curious and interesting manner, 
which at the same time has proved of very great practical 
utility. Very many of the products resulting from the 
expulsion of a-rays, although after their formation they 
are either not at all volatile or can only be volatilised 
at a high temperature, yet at the moment of production 
behave like volatile substances, and are carried away 
under suitable circumstances from the preparation in 
which they are produced, and deposited on the nearest 
available surface. The best conditions are obtained by 
working in a good vacuum, and charging the preparation 
positively, and the surface, on which it is required to 
deposit the recoil product, negatively. The residual 
atom, after the a-particle is expelled, carries a positive 
charge, and so is attracted to the negatively charged 
surface. It is essential that the preparation should 
be in the form of a very thin layer in order to give the 
recoiling product a chance of escaping from it. In this 
way many products, of period of life too short to allow of 
their being separated by any other method, have been 
isolated and identified with ease. 



Questions of Nomenclature. 

The question, How can an element or the atom of an 
element change? has given rise to many arguments, of 
etymological rather than scientific importance. What 
we now certainly know, and what radioactivity has 
given us for the first' time the opportunity of learning 
is, first, that some elements do change, and secondly, 
how they change. The element radium changes, by 
the loss of an atom of helium, into the efnanation, which 
is about as different from radium in its chemical or 
material nature as two elements well could be. The 
one is a member of the group of alkaline-earth, the other 
of the argon family of elements. 

After all, is not this rather to be anticipated ? When 
we arrange the elements in order of their atomic weights 
— an arrangement which led to the recognition of what 
is known as the Periodic Law (Fig. 43, p. 214) — the most 
sudden and surprising differences appear between suc- 
ceeding elements. Chlorine, potassium, and argon are 
three succeeding elements in such an arrangement, and 
there is no resemblance whatever between them. In 
the nine successive transformations radium undergoes, 
the atom suffers, in most but not in all, a disintegration 
in which a helium atom is expelled. The heavy residues 
of the original atom remaining after the successive loss 
of one, two, three and so on of these helium atoms 
constitute the intermediate bodies — the emanation, 
radium A, radium B, and radium C — successively 
produced, each from the preceding. It is therefore 



rather to be expected that the succeeding transition- 
substances produced one after the other should differ 
entirely from one another in their material character- 
istics. Further discoveries on this important question 
are dealt with in Chapter XV. 

Definition of the Atom. 

Let us from the point we have gained now face 
the question, which has proved a difficulty to so many, 
of how it is we find that the elements and the atoms 
are actually changing. The word atom is, of course, 
derived from the Greek, and at first meant the indivisible 
or the undivided. For a long time it had a subjective 
meaning only, being the smallest particle imaginable, 
rather than the smallest particle obtainable, and as 
such it belongs to metaphysics, not to physical science. 
The idea of the atom was first given an objective mean- 
ing by Dalton. * He showed that chemical change be- 
tween two elements occurs in definite proportions by 
weight of the two elements. If unit weight of one is 
taken, the weight of the other will have a definite fixed 
value. But often the same two elements unite to form 
more than one compound in different proportions. 
Then, if unit weight of the one is still taken for reference 
throughout, the ratio of the weights of the other in 
various compounds will be simple multiples or sub- 
multiples of one another, indicating that elements do 
not combine in haphazard proportions, but " atom for 
atom" by fixed increments or units of combination 
having definite relative weight. Thus, one atom of 
carbon combines with either one or two atoms of oxygen, 
and for iron and oxygen the ratio is either one to one or 
two to three. These units of chemical combination 
of definite relative weight are the atoms of the chemist. 
In all the various changes of matter which chemistry 
has investigated it has sufficed to regard all combination 
as taking place atom by atom, and fractions of an atom 
or the subdivision of atoms has not been necessary. 


In compounds the component atoms preserve their 
individuality and identity, because compounds can 
always be decomposed to give back the same elements 
out of which they are formed and not new ones. In 
none of these changes does any deep change of the com- 
ponent atoms themselves take place. As chemical 
changes till recently were the most fundamental material 
changes known, the chemist's atom fulfilled in a derived 
sense the ancient meaning of the smallest particle that 
exists. It did not suffer subdivision in the most funda- 
mental changes known. But in this sense its meaning 
was coupled with that of the particular element to which 
it referred. Thus the atom of uranium is about 240 
times as massive as the atom of hydrogen. An atom 
of uranium is the smallest particle of uranium which 
exists. An atom 240 times lighter than this is known, 
but it is not uranium, it is hydrogen. 

Elements and Chemical Compounds. 

The discoveries in radioactivity have left this meaning 
of the word atom unchanged. The atom of radium is 
the smallest particle of radium that exists, and is the 
unit of all the chemical changes radium undergoes. 
When, by new and more fundamental changes than those 
before known, it changes, it is no longer an atom of 
radium. The matter formed is as unlike radium as any 
body well could be. You may, if you like, regard the 
radium atom as a compound of the atom of emanation, 
and of the helium atom which result on its disintegration, 
as it certainly is such a compound, but you must make it 
quite clear that you do not mean a mere chemical com- 
pound, which may at will be formed from and decom- 
posed into its constituents. Were radium a chemical 
compound of helium it would, as Sir William Huggins 
has pointed out, show the spectrum of helium. Instead, 
it shows an entirely new spectrum, clearly analogous to 
but distinct from that shown by barium, its nearest 
chemical relative. The spectrum of helium is not shown 


until after the radium has disintegrated. The radium 
spectrum does not contain a single helium line. 

The most vital distinction, however, between an 
element and a compound in the chemical sense is this: 
both are ultimately compound. Of that there can be 
now no doubt. But the energy change which attends 
the resolution of an element into its constituent parts 
is of an order of a million times greater than in the case 
of the resolution of any chemical compound. Although 
this is a question of degree, it is of a degree of so entirely 
different an order of magnitude that it completely 
differentiates the two types of complexes, and nothing 
but confusion can result from giving to each the same 
name. Radium is as much an element as any of the 
other eighty. If radium is complex, so, almost certainly, 
are all to greater or less degree. If radium changes, 
so may (perhaps even so do) all. Their complexity is of 
a completely different character from that of chemical 
compounds, and it is best in the end to retain the old 
words " atom " and " element " in the sense they have 
had since the time of Dalton rather than attempt to 
meddle with this traditional, and to scientific men, 
well-understood nomenclature. The atom of the chemist 
remains exactly what it was. Why, therefore, alter its 
name ? If you call it a molecule, how are you to dis- 
tinguish it from the chemical molecule, which has also 
its own definite meaning distinct from the chemical 
atom ? 

The Experimental Facts. 

These questions of nomenclature at first diverted 
attention from the experimental fads, and gave rise 
to much more or less random criticism of the younger 
workers in radioactivity. Another source of con- 
fusion has been the tendency to associate the discoveries 
in radioactivity with other entirely distinct discoveries 
made somewhat earlier with reference to the nature of 
the negative electron. 

It was thought at one time that it would be possible 


to explain the atoms of matter as being built up entirely 
of electrons or atoms of electricity, which turned out 
to be as little in accord with actual evidence as it would 
be to regard the solar system as composed entirely of 
planets and to neglect the central sun. The problem 
of the real nature of the atoms of matter has not been 
completely solved by either of these independent 
scientific advances. 

Another objection to the validity of radioactive 
evidence has been the minuteness of the amounts of 
matter on which the evidence is based. 

It has been stated that it is impossible to come 
to any settled conclusions in regard to radioactivity, 
until enough of the materials can be obtained to suffice 
for the requirements of chemical investigation. But 
surely, this criticism puts weight on mere familiarity 
with the older methods rather than on their real in- 
trinsic value. The tests by which we can recognise and 
identify with ease, and measure with accuracy the 
amount of, say, one billionth of a milligram of the radium 
emanation, possess a philosophical foundation which 
would challenge comparison with any of the tests of the 
chemist on any kind of matter, in any quantity great 
or small. 

The Nature of Atomic Disintegration. 

It is my intention to give you, so far as I am able 
with accuracy, broad general mental pictures of radio- 
active processes, rather than the detailed technical 
investigations on which these pictures are based. Bear 
in mind exactly the relation of such mental pictures to 
the discovered facts. The pictures may not be true, 
but they are not demonstrably false at the present time. 
That is to say, you may in any case, without fear of 
being led into error, apply the picture you have to what 
is taking place, and the view will lead you to expect 
certain consequences, and these consequences in every 
known case agree with the facts. Without such mental 


pictures, or generalising hypotheses, no man could 
encompass even a small part of one science. So long 
as the deductions from the hypothesis are in agreement 
with facts and can be used to predict them accurately, 
even when they are still unknown, thus saving the 
memory, the hypothesis or mental picture is not even 
supposed or expected to be the absolute truth. So long 
as all the known facts occur as though the hypothesis 
were true, the latter serves a very useful purpose, 
although at any time it may be replaced by a deeper 
view, one step nearer to absolute truth. 

In the early history of the subject two possible alter- 
natives had to be taken into account with reference to 
the exact nature of radioactive changes. Radioactivity 
is an atomic phenomenon, and the radio-elements are 
slowly undergoing changes. What do we mean by 
" slowly" in this connection ? Two possibilities arise. 
Either the slow changes may result from a slow gradual 
alteration, through all the atoms of a radioactive sub- 
stance gradually evolving their stores of internal energy 
and changing by slow degrees into new kinds of matter. 
This point of view it was never possible to entertain for 
a moment. Or, the change is slow and gradual with 
regard only to the mass of the substance as a whole, but 
sudden and explosive in character with regard to each 
individual atom as its turn to disintegrate arrives. 
This, from the first, the only possible point of view, is 
in accordance with all that has since been discovered with 
regard to the nature of the successive disintegrations 
and of the a-rays expelled. Radioactive changes 
proceed in cascade, from step to step, the accomplish- 
ment of each successive step taking on the average 
a definite time. But as regards the individual atom 
disintegrating, the change is sudden in time and of 
the nature of an explosive disruption, in which an 
a-particle is expelled with enormous speed, and the 
old atom becomes ipso facto a new one, of atomic 
weight four units less. Regarding the individual 
radium atom, for example, there is no gradual change 


into the emanation and helium atoms. Regarding the 
whole mass of radium, there is a very gradual change 
in the sense that some definite small proportion of the 
whole suffers disintegration in each unit of time. 

The Chance of Disintegration. 

This, then, is the very vivid mental picture of atomic 
disintegration which the detailed researches in radio- 
activity have established. Any one radio-element like 
radium being considered at any instant, among its 
innumerable host of atoms, most of which are destined 
to last for hundreds, some for thousands of years, a 
comparatively very small proportion every second fly 
apart, expelling a-particles and becoming emanation 
atoms. Next second the lot falls to a fresh set to dis- 
integrate, and so the process goes on, a-particles being 
expelled as a continuous swarm, and yet so small a 
fraction of the whole changing that the main part of 
the radium will remain unchanged even after hundreds 
of years. Now consider the emanation atoms formed. 
These are much less stable than the atoms of radium. 
A much larger fraction of these disintegrate every second, 
producing more a-particles and a new body not yet 

It is now necessary to consider briefly the exact 
nature of radioactive change and the laws it follows. 
The deduction of these laws is a matter for the mathe- 
matician. We are chiefly concerned with the general 
conclusions which have transpired. I will first state 
the most important of these in words divested of mathe- 
matical symbols. The chance at any instant whether 
any atom disintegrates or not in any particular second 
is fixed. It has nothing to do with any external or 
internal consideration we know of, and in particular 
it is not increased by the fact that the atom has already 
survived any period of past time. The events of the 
past in radioactive change have, so far as we can tell, 
no influence whatever on the progress of events in the 
future. This follows from the consideration of the one 


general mathematical law which all known cases of 
atomic disintegration so far investigated have been 
found to follow. Fortunately the law itself is simple. Its 
application in individual cases is often complicated, but 
I shall confine myself to the simplest, which are at the 
same time the most generally important, consequences. 
The chemist has to do with many types of change all 
following different laws. In some the rate of change — 
that is, the quantity of the substance changing in the 
unit of time — is proportional to the quantity present of 
the substance which is changing, in others to some power 
of this quantity. Now, in radioactive change the rate 
of change is invariably simply proportional to the quan- 
tity of changing substance. This seems easy enough, 
but I would warn the uninitiated that they must not 
overlook the important fact that since the quantity of 
a changing substance itself changes as time goes on, 
owing to the progress of the change, the rate of change 
being proportional to the quantity also continuously 
changes, and at no time has a constant value. Hence 
you cannot get much further by simple arithmetic and 
algebra. Of course, in the case of a slow change like 
that of radium itself, when even in a lifetime the quan- 
tity of radium is not very appreciably reduced by the 
operation of the change, it is allowable to neglect the 
slow alteration of the rate of change with the time and 
to consider the rate of change as constant, since for short 
periods of time it essentially is so. In most cases some 
knowledge, withal a slight one, of the mathematics of 
continuously varying quantities is essential for the 
complete deduction of the laws of radioactive change. 
However, as my intention is to avoid mathematics, 
I shall simply state these consequences ex cathedra. 

The Period of Average Life of a 
Disintegrating Atom. 

The rate of change in any single case of atomic 
disintegration is proportional to the quantity of the 
substance which is changing. The usual plan is to let 


the symbol \ represent the fraction of the total changing 
per second, and to this symbol X is given the special 
name " the radioactive constant." X may represent 
a small or a large fraction, according to the particular 
case, according as the disintegration process is slow 
or rapid. The important point is that it is a real con- 
stant of nature in every case, independent of the past 
and future history of the substance, its actual amount 
whether large or small, and of every other consideration 
whatever. Thus for the emanation of radium, X, the 
radioactive constant, has the value 1/481,250, which 
signifies that in this case 1/481, 250th of the total amount 
of emanation in existence changes per second. The next 
step, skipping the mathematics,-' is that the average period 
of life of the atom of a radioactive substance — that is 
to say, the period of time in seconds it exists on the 
average before its turn comes to disintegrate — is simply 
the reciprocal of the radioactive constant, or 1/X. 
Thus the average life of the radium emanation is 481,250 
seconds, or 5-57 days. 

Now as radioactive change proceeds during every 
instant at the rate proportional only to the total quan- 
tity of substance undergoing the change, which is 
present and remains unchanged at that instant, and as 
in this method of looking at the changes we do not 
consider at all the absolute quantities, only the fraction 
of the whole changing, it follows that X is always of the 
same value throughout the process from start to finish. 
It also follows that l/X, the period of average life of 
the remaining atoms, does not, as you might be inclined 
to suppose, tend to lessen as time goes on. The atoms 
disintegrating first have a far shorter period of life, and 
those disintegrating last have a far longer total period 
than the average. But at any instant throughout, con- 
sidering only the atoms still remaining unchanged at 
that instant, then from that instant the average period 
of life is always l/X. 

1 So far as I know, the period of average life was first deduced by 
Mr. J. K. H Inglis, to whom I put the problem. 


Our own period of average life, of course, follows 
very different and far more complicated laws. The 
expectation of life at any age is a practical problem for 
the actuary. But every one knows, owing to the mor- 
tality among infants, that the expectation of life at 
birth is less than shortly afterwards, when it reaches a 
maximum and then gets less and less with increasing age. 
The " expectation of life " of a radioactive atom is 
independent of its age — as it happens the simplest 
possible law and one lending itself, as will appear, to 
some most beautiful deductions. That this is so can 
be directly proved in the simplest way, by comparing, 
for example, the rate of change of newly-born radium 
emanation, not in existence a few minutes before, with 
that of the residue of an originally much larger quantity 
that has survived a period several times greater than the 
period of average life. 

The Unknown Cause of Disintegration. 

This answers fully the general question. How does 
an element change ? You will probably wish to know 
why it changes in this particular way. That cannot 
be said, although the true answer would undoubtedly 
take us far. All that can be stated is that the immediate 
cause of atomic disintegration appears to be due to 
chance. If the destroying angel selected out of all 
those alive on the world a fixed proportion to die every 
minute, independently of their age, whether young or 
old, if he regarded nothing but the number of victims 
and chose purely at random one here, and one there, to 
make up the required number, then our expectation 
of life would be that of the radioactive atoms. This, 
of course, is all that is meant by the statement that the 
course of atomic disintegration appears to be due to 
the operation of " chance." 

It is natural to inquire why this particular law is 
followed. On this fundamental question no light is 
yet forthcoming. There is always " a cause of the 


ultimate cause." Atomic disintegration is assuredly 
the ultimate cause of radioactivity. It does not 
weaken this deduction that as yet we have not found 
the ultimate cause of atomic disintegration. Various 
possible causes have been discussed. Most of them, 
so far from helping the elucidation of the " why," do 
not conform even to the " how." The law of radio- 
active changes shows clearly that the past history of an 
atom does not increase its chances of undergoing dis- 
integration in the future, which is a fundamental step 
gained, although it leaves the ultimate problem unsolved. 
There is another way of stating the law of radio- 
active changes, and that is by saying that as the time 
increases in arithmetical progression the amount of 
substance remaining decreases in geometrical progres- 
sion. Suppose in a time of r seconds one-half of the 
total amount changes and one-half remains unchanged. 
In the next period of t seconds, 2 r altogether, one-half 
of what is left — that is, one-quarter — changes, and one- 
quarter of the total remains unchanged. In 2 r the 
quantity is reduced to 1/2^. In any period of time 
represented by N r seconds, where N is any multiple or 
submultiple, the quantity of substance remaining is 
1/2". It remains to state what relation the time r 
required for the half-change to occur, bears to the period 
of average life l/X of the former way of considering the 
change. There is a fixed ratio between these two 
periods, the latter being always 1-45 times the former. 
In a time equal to the period of average life l/X, the 
quantity of substance present is reduced to l/e=0-368 
of the initial quantity. 

Determination of the Period of Average Life. 

These considerations would have little interest to 
us but for the fact that they afford the means whereby 
the period of average life of any radioactive element can 
by their aid be exactly determined, not only for those 
transition-bodies like the emanation, which change so 


rapidly that we can watch their complete transformation 
in the course of a few days or weeks, but also for the 
primary radio-elements, some of which we know require 
thousands of millions of years to run their course of 
change. The average life of a radioactive element, 
representing as it does a fundamental constant of nature, 
is one of its most important attributes. Our own 
period of average life being strictly limited, it naturally 
affects very much our way of looking at the various 
radioactive bodies. If, for example, the average life 
is a matter of a few days, as in the case of the radium 
emanation, we regard the body as an ephemeral transi- 
tion-form. If it is, as in the case of radium, a few 
thousand years we are inclined to look upon the sub- 
stance as a permanent and primary radio-element. 
There is really not this sharp difference. But it is con- 
venient to divide radioactive bodies into two classes, 
and in the one to put those for which the periods of 
average life are short compared to our own, and in 
the other to put those for which the periods are long. 
The method employed to determine the value of this 
fundamental and all-important constant is naturally 
quite different in the two cases. In the first, simple 
direct observation suffices. Thus if we measure the 
decay of the activity of any separated quantity of the 
emanation of radium with time, we shall find that it 
decays in a geometrical progression with the time to 
half its initial value in the course of 3-84 days. The 
period of average life is 1-45 times greater, or 5-57 days. 
But in the case of a body, of which one thousandth, or 
one thousand-millionth as the case may be, changes 
annually, simple direct observation does not help much. 
How are we to proceed ? 

In the first place, let us consider the cases of uranium 
and radium. We may determine how many times 
more powerfully radioactive radium is than uranium. 
The radioactivity of radium is several million times that 
of uranium when the a-rays of equal quantities of the 
two elements are compared. From this it may be con- 


eluded that the period of uranium is several million times 
longer than that of radium, and if the latter is known, 
that of uranium may be roughly estimated, although 
it is a period of some thousands of millions of years. 

As a matter of fact, there is a very beautiful generalisa- 
tion, I have already referred to briefly, and which 
later on I shall try to develop further by the aid of an 
analogy, by means of which the periods of average life 
of the radio-elements of the second class, those, that is, 
which are long-lived compared with ourselves, have 
come into the region of exactly knowable quantities. 
If the period of average life of a single member of a 
series of successive atomic disintegrations is knoAvn 
the others can be calculated, provided certain data, not 
entirely impossible to obtain, are known. It will clear 
the ground considerably if I attempt to give you the 
main idea succinctly in the case of radium itself and of 
the first product of its disintegration, the emanation of 
radium. I have already alluded to the fact that owing 
to the very rapid disintegration of the emanation its 
quantity does not continuously accumulate, but reaches 
an equilibrium ratio with respect to the radium pro- 
ducing it, in which the amount of already formed 
emanation disappearing is exactly counterbalanced 
by the amount of new emanation formed. 

The Period of Average Life of Radium. 

This state of things is known generally by the name 
of radioactive equilibrium. The importance of the 
existence of this state of radioactive equilibrium it is 
impossible to overrate. Many problems, as we shall 
come to see, which, to us with our limited period of life, 
might well appear absolutely insoluble, connected as 
they are with periods of time so vast that our little 
life by comparison appears a mere moment, are solved 
directly by the proper application of this principle. Now 
I am only giving you the main idea and one specific 
illustration of what is in fact a law of great generality. 


, By the law of radioactive change, if X^ is the radio- 
active constant of radium — i.e., the fraction of the 
whole changing per second — and N is the total number 
of radium atoms dealt with, then the number of radium 
atoms changing into the emanation per second, and 
therefore also the number of atoms of fresh emanation 
produced per second, is XiN. But in equilibrium this 
equals the number of emanation atoms disappearing. 
If the radioactive constant of the emanation is X2, and 
the number of atoms of emanation present during 
equilibrium is denoted by X, the number of emanation 
atoms disappearing per second is X2X. Hence we have 

\,N=\n'K and i:T=r^« 
^ ^ N X2 

This law, the most important in radioactivity, thus 
states that in successive disintegrations the product 
accumulates in quantity until a fixed ratio with respect 
to the parent body is attained, and this ratio is inversely 
proportional to their respective radioactive constants 
or directly proportional to their respective average lives. 
It is necessary for the law to hold true that the period 
of the parent body should be much longer than the 
periods of any of its products, and in this case the product 
selected need not necessarily be the first product, but 
may be any one of the successive products formed in 
the series. 

X2 is well known by direct observation. Now if 
X/N, the ratio between the number of atoms of emana- 
tion and of radium in equilibrium together, can be 
found, then \i, the radioactive constant and therefore 
1/Xi, the period of average life of radium can be de- 
duced. That is the important thing — the period of the 
average life of radium, the rate at which it is changing, 
and a host of vitally important consequences, can be 
deduced. For a slowly changing body like radium 
the second is an inconveniently short unit of time to 
employ, and it is better to take a year. What is wanted 
is the fraction of any quantity of radium which changes 


in a year. The quantity X/N, which is the ratio of 
the number of atoms of emanation and of radium in 
equiUbrium together, can be deduced by ordinary 
physico-chemical laws if the actual volume of emanation 
in equilibrium with a given quantity of radium can 
be determined. As already mentioned (p. 82), this 
volume was first approximately measured by Sir William 
Ramsay and myself in 1904. The actual volume of 
emanation is excessively minute, but it is just within 
the range of measurement. From our results we con- 
cluded about l/l 150th part of the radium changes 
annually, so that the period of average life on this 
estimate is 1,150 years. Owing to the excessive minute- 
ness of the volume, the method is not an accurate one, 
tending, since the volume of emanation is likely to 
be too great unless every trace of other gas is absent, 
to give too short a period. Later experiments by the 
same method with much larger quantities of radium 
have shown that the correct value is about double that 
first found. With the gro^vth of the subject other 
methods, less direct but more accurate, have become 
available. Professor Rutherford recently, from a con- 
sideration of a large number of separate data 
accumulated by himself and others bearing on this 
question, came to the conclusion that the period of 
average life of radium is not very far removed from 
2,500 years, and we shall take this value as the most 
probable. It may suffer slight further alteration as 
fresh data are accumulated, but it is very improbable 
that it is seriously in error. Within narrow limits 
the average life of radium may be taken to be 2,500 

The Total Energy evolved in the Complete 
Disintegration of Radium. 

A knowledge of this important constant enables 
us at once to say how much energy any quantity of 
radium would evolve in the course of its complete 


change — that is, during a period of some thousands of 
years. We saw (p. 22) that a gram of pure radium 
evolved about 133 calories of heat per hour. There are 
8,760 hours in the year, so that in a year a gram of 
radium evolves about 1,160,000 calories. In a year 
1 2500th part changes. Therefore in the complete 
change of one gram of radium no less than 2,900,000,000 
calories would be evolved. The energy evolved in the 
change of radium is nearly a million times greater than 
that evolved from a similar weight of matter undergoing 
any change known previously to the discovery of radio- 
activity. By the burning of a gram of coal, for example, 
only about 8,000 calories are obtained. In this change, 
however, 2| grams of oxygen are also consumed, so 
that per gram of the two substances taken together the 
heat evolved is only 2,200 calories. On this basis 
of calculation the energy of radium is well over a million 
times that furnished from the combustion of coal. 
No wonder then that to account for the boundless energy 
displayed everywhere in the starry heavens proved a 
difficult problem for physicists, acquainted with no 
more energetic chemical process than the burning of 
coal ! 



Why has Radium Survived ? 

One of our chief duties will be to follow out this theory 
of the disintegration of atoms in radioactivity. The 
bare idea of elements spontaneously changing raises so 
many obvious and apparently insurmountable difficulties 
that it will be interesting to consider them as they arise 
and to consider what answer can be made to them. 
To-night we must concentrate on one of the chief of 
these — a difficulty which no doubt has already pre- 
sented itself in many of your minds. If radium is 
changing at the rate of nearly one two-thousandth 
part every year, how is it that there is any radium 
left at the present time ? Even at the beginning of 
the time recorded in past history there must have 
existed several times as much radium as there is now, 
if the rate of disintegration has been constant over that 
period, while a hundred thousand years ago it can be 
calculated that there must have existed a thousand 
billion times as much as to-day, had the steady disin- 
tegration been going on at its present rate. That is to 
say, even if the whole world were originally pure radium, 
in a period of time brief compared to that which we 
know from geological evidence it has actually been in 
existence, there would be practically none left, and 
certainly not as much as actually exists to-day. Or, 
looking forward instead of backward, if we put this half- 
grain of radium bromide in a safe place, and then could 
revisit the earth say twenty-five thousand years hence, 
we should find less than one-thousandth part of it 



remaining. The slow disintegration would have done its 
work and changed the radium into the non-radioactive 
elements which are being formed from it. This question, 
apparently so insoluble, in reality admits of the most 
direct and satisfactory answer on the disintegration 
theory and serves as a good example of how a theory, if 
it is worth the name, must be able to predict future 
discovery as well as to explain the existing facts. 

An analogy to facts we have already discussed will 
help us to find the solution of this difficulty. In the 
emanation of radium we have become acquainted with a 
body changing so rapidly that at the end of a month none 
of the original quantity remains. How is it there is 
any emanation in existence at all ? Because it is being 
reproduced as fast as it disappears. Is there any 
reproduction of radium going on, balancing the effect 
of its disintegration and maintaining its quantity from 
age to age ? Radium is the direct parent of the emana- 
tion. Itself changing more than a hundred thousand 
times slower than its product, it maintains the quantity 
of emanation in existence over a period a hundred 
thousand times longer than would otherwise be the 
case. Is there then a parent of radium ? Does there 
exist any other element producing radium by its own 
disintegration as fast as that already in existence 
disappears ? 

The Reproduction of Radium. 

Do not regard this thirty milligrams of radium 
bromide as something merely by itself. Consider its 
history. By infinite labour and patience this tiny 
quantity of radium has been separated from several 
hundredweights of the mineral pitchblende. Suppose 
in this operation all the rest of the mineral, after the 
extraction of the radium, were preserved and put in a 
safe place. When we revisited our specimen of radium 
twenty-five thousand years hence, and found practically 
none of it remaining, should we find that the mineral 
from which it was extracted had in the meantime grown 


a fresh crop of radium ? The answer is that we 

This was one of the first predictions made from the 
theory of atomic disintegration and one of the most 
recent to be confirmed by experiment. Long before 
the data were available which enabled an exact estimate 
of the life of radium to be calculated, it was recognised 
that radium, though at first sight a permanent and 
primary radio-element, is changing so rapidly that, had. 
there existed no process in which fresh radium is supplied 
to replace that changing, none could possibly have 
survived till the present day, and from general principles 
it was possible to make a shrewd prediction as to which 
element was the parent of radium. We have already 
considered the general principles which enabled the 
prediction that helium was one of the ultimate products 
of radioactive changes to be made. Ultimate products 
must co-exist with the radio-elements producing them 
in all the natural minerals in which the latter are found. 
Something of the same reasoning applies to the parent 
of radium, only in this case it is far more definite and 
elegant. The parent of radium must co-exist with 
radium in all minerals in which radium is present. Now 
it is at once obvious, if this explanation of the parent of 
radium is to meet the case, that such a body must be 
changing very much more slowly than radium, otherwise 
there would arise the same necessity to assume the 
existence of a parent of the parent as there is of a parent 
of radium. The original first parent of radium must be 
changing excessively slowly to maintain a steady supply 
of radium over long epochs of geological time. 

The Ratio between the Quantities of Uranium 
AND Radium in All Minerals. 
By the law already formulated on p. 118, in two succes- 
sive, not necessarily consecutive, disintegrations of which 
the second is much more rapid than the first, the more 
rapidly changing body accumulates in quantity until 
a fixed ratio with respect to the parent body is attained, 


and this ratio is inversely proportional to the ratio of 
their respective rates of change, or directly proportional 
to the ratio of their respective periods of average life. 
Let us apply this law. The parent body is the parent 
of radium. The quantity of radium in minerals must 
therefore attain a fixed ratio with respect to the quantity 
of the parent of radium, and this ratio is the ratio of 
the period of average life of radium to that of its parent. 
The quantity of helium that accumulates in a mineral 
continually increases as time goes on, assuming the 
helium does not succeed in escaping, and no definite 
proportion between helium and radium is to be ex- 
pected. But the case is different with radium and its 
parent. There must be a fixed ratio, independent of 
the age of the mineral examined. As the original 
first parent of radium must be changing excessively 
slowly to survive geological epochs of past time, there 
must be always a very large quantity of it in the mineral. 
As the radium is changing, from the standpoint of 
geological epochs of time, very rapidly, there must 
always be a very small quantity of radium. Between 
these quantities great and small there must exist the 
same ratio as between the respective periods of average 
life of the two bodies. 

A very cursory examination of the minerals in which 
Mme. Curie found radium was sufficient to point strongly 
to the probability that uranium is the primary parent 
of radium. Uranium was, as we have seen, the original 
element for which the property of radioactivity was 
discovered, and its radioactivity is several million times 
more feeble than that of radium. Now the radioactivity 
depends only on the atoms actually breaking up, and 
therefore in comparing uranium with radium it follows 
that uranium must be disintegrating several million 
times more slowly even than radium, so that if uranium 
produces radium the quantity of uranium must be 
several million times greater than the quantity of 
radium in minerals. But this is exactly what Mme. 
Curie found to be the case in the minerals she worked 


up for radium. So that from the very first there existed 
a strong presumption that uranium is the original parent 
of radium. The evidence in support of this view at 
the present time is complete and satisfactory. We owe 
it to the careful work of McCoy, Strutt and Boltwood 
that the genetic relation between uranium and radium 
has been established. They determined the ratio 
between the quantities of uranium and of radium in a 
large number of minerals. In every mineral examined 
containing uranium there was found to exist a direct 
proportionality between the quantity of uranium and 
that of radium. To Rutherford and Boltwood together 
we owe the exact determinations of this important con- 
stant of proportionality. They found that for every 
one part of radium there always exists 3,200,000 parts 
of uranium. This constant gives directly, unless other 
undetermined factors interfere, the ratio of the average 
lives of the two elements. As we have seen, that of 
radiurji is 2,500 years. Hence it follows that that of 
uramum is 8,000,000,000 years. Enormous as this 
period is, it is not now merely a deduced or calculated 
value. I obtained the same result by direct experiment 
from the rate of production of helium from uranium. 

Hydraulic Analogy to Radioactive Change. 

It will help us considerably if we try to find some 
analogy to the important and intricate relations that 
exist between uranium and radium. We may take for 
illustration the magnificent system of waterworks which 
supply this city, which we will suppose have been given 
over to us by the Corporation to control for the purposes 
of our illustration. As you know, we in Glasgow are 
supplied ultimately from Loch Katrine through an 
intermediate reservoir at Milngavie. We shall first cut 
off Loch Katrine from all fresh sources of supply of 
water, and from all outlets except to the intermediate 
reservoir at Milngavie, and we shall see to it also that 
the latter receives no water except from Loch Katrine, 



and delivers none except to Glasgow. We shall then 
issue to our engineers the instructions that there must be 
delivered every hour at Milngavie from Loch Katrine 
approximately one eight-millionth part of the total 
store of water in Loch Katrine, and from Milngavie to 
Glasgow every hour one two-thousand-five-hundredth 
part of the total store of water at Milngavie. Then, if 
instead of hours we read years, the quantity of water 
in Loch Katrine represents the quantity of uranium, 
and the quantity of water in Milngavie that of radium. 
For the sake of brevity we shall term Loch Katrine the 
source and Milngavie the reservoir. 

First we shall suppose that our regulations have 
been in operation already a considerable number of 
hours, as this is the condition in which, reading years 
for hours, we find uranium and radium together in 
minerals in Nature, for example, in a piece of pitchblende. 
What relation will the quantity of water in the source 
bear to the quantity in the reservoir — that is, the quan- 
tity of uranium to the quantity of radium ? The amount 
of water the reservoir receives is quite independent of 
the amount it contains, but the amount it delivers is 
proportional to the amount it contains. Similarly the 
amount of radium produced from uranium does not 
depend at all on the amount of radium already present, 
while the amount that itself changes depends only 
on and is proportional to the amount present. Never- 
theless, we shall find that there is about three million 
times more water in the source than in the reservoir. 
Because only under this condition is the intake of the 
reservoir equal to the outflow from the reservoir — that 
is, the production of new radium equal to the disappear- 
ance of the old. Imagine, for example, that there was 
just twice as much water as this ratio in the reservoir, 
then twice as much would flow out as flows in, and the 
supply in the reservoir would be rapidly depleted. Or, 
if there were but one half as much in the reservoir, 
twice as much would flow in as out, and the supply in 
the reservoir would increase. In either case, intake 


and outflow would ultimately become equal, and no 
further change would then occur until both the source 
and the reservoir were empty. But let us now dis- 
connect Loch Katrine from Milngavie reservoir, which is 
equivalent to separating, as Mme. Curie did, the radium 
from the uranium in pitchblende. Obviously the 
reservoir by itself will now be able to supply water for 
a very much shorter time than it did before, and, in 
general, with the conditions stated, source and reservoir 
together will last three million times longer than the 
reservoir alone. The radium on the table will have half 
disintegrated, so that only half will remain, in about 
1,700 years. Whereas had it remained in the mineral 
associated with its parent uranium, the quantity of 
radium in the mineral will not be reduced to one half 
what it is now until 5,000,000,000 years have elapsed. 

The Age of Pitchblende. 

Thus we can say, following a cautious reservation 
once made by Professor Tait, provided the causes 
that are now at work have always been in continuous 
operation in the past as they are now, and that we know 
of all the causes that have been at work, 5,000,000,000 
years ago there must have been about twice as much 
uranium and radium in this piece of pitchblende as 
there is to-night. Since, however, there is actually 
in this pitchblende now over 50 per cent, of uranium, it 
is not possible that it can have been in existence in its 
present form more than 5,000,000,000 years. But, 
even from a geological point of view, this is a very long 
period of time indeed; longer, perhaps, than it would be 
profitable in the present state of science to push back 
our inquiries. That, then, is the position with regard 
to the maintenance of radium in Nature. Even when 
we deliberately leave out of account the possibility there 
may exist in Nature entirely unknown processes re- 
plenishing the supplies of uranium, just as there are 
replenishing Loch Katrine, there is no difficulty in 


accounting for the continuous maintenance of radium 
over a period of the past as great as, or greater than, 
there is any reason to beUeve the earth has been in 
existence in its present condition. This is as far as we 
need pursue our analogy for the moment, but we shall 
again find it useful at a later period. We must pass 
on to another aspect of the question. 

Uranium X. 

At this stage it will be well to make a short digression 
into the radioactivity of uranium itself, and how it 
is explained on the theory of atomic disintegration. 
Uranium and its compounds in their normal state give 
out both a- and /S-rays. As in all other cases, the yS-rays, 
being photographically the most active and being the 
more penetrating, were the first chiefly studied. Sir 
William Crookes and also M. Becquerel found that by 
certain chemical processes a new substance in minute 
quantity could be separated from uranium, to which 
Crookes gave the name uranium X, and this new body 
produced the whole of the photographic activity of 
uranium. The uranium after this treatment no longer 
affected a photographic plate. Crookes concluded that 
the radioactivity was due in reality to the presence 
of the foreign substance in minute amount, which he 
called uranium X, and that pure uranium was not 
radioactive. I repeated these experiments, and found 
that only the y8-rays of uranium belonged to the uranium 
X. Uranium freed from uranium X gave its normal 
amount of a-rays. Then it was found that the y3-radia- 
tion of uranium X decayed steadily in a geometrical 
progression with the time, whereas the uranium that 
had been freed from uranium X and at first gave no 
/S-rays, gradually and completely recovered its power 
of producing /3-rays. Uranium grows uranium X, in 
exactly the same way as radium grows the emanation. 
The activity of uranium X after separation from uranium, 
consisting entirely of /3-rays, steadily decays in a geo- 


metrical progression with the time, faUing to one half 
the initial value in 24-6 days. The average life of 
uranium X is thus 35-5 days. 

The disintegration of uranium up to the point so far 
discussed is represented on the following scheme: 

^ ( 234 J — ) 

Uranium. Uranium X. 

8,000,000,000 years. S5-5 days. 

Fig. 28. 

This is as far as the methods of radioactivity enable 
us directly to trace the disintegration of uranium at 
the present time. Thp substance produced — uranium 
X — is only an ephemeral transition-form, lasting on the 
average 35-5 days, and when it disintegrates, the process 
appears to come to a stop so far as our experimental 
methods have yet been able to disclose. 

Now, on the view that has been developed that 
uranium is the parent of radium, it is natural to suppose 
that uranium X in the course of time turns into radium. 
A little consideration will show that if this were the case 
it might easily be overlooked at first on account of the 
very long period of life of radium compared with that of 
uranium X. As already explained (p. 92), chemical 
and spectroscopic methods of detecting matter depend 
only on quantity, but radioactive methods depend upon 
quantity divided by life. Assuming equal effects 
produced in the disintegration of an atom of uranium X 
and of an atom of radium, since the life of the latter is 
30,000 times that of the former, it will be necessary 
to have 30,000 times as much radium as of uranium X 
to produce equal radioactive effects. 

Attempts to detect the Growth of Radium. 

In 1903 I started a series of special experiments 
which have been continued ever since, partly in con- 


junction with Mr. T. D. Mackenzie and more lately with 
Miss A. F. Hitchins, to see whether uranium does, in 
fact, produce radium. The uranium, after being puri- 
fied as completely as possible by chemical methods from 
radium, is left sealed up in a flask and is periodically 
tested to see if a growth of radium has occurred. The 
method of testing for minute traces of radium is a very 
simple and accurate one, allowing quantities of radium 
of only a few million-millionths part of a gram to be 
detected with certainty and measured with exactitude. 
Use is made of the characteristic emanation generated 
by radium. Uranium does not generate any emana- 
tion. The uranium solution to be tested for radium, 
after standing sealed up in a glass flask for a period of 
at least a month to allow the equilibrium quantity of 
emanation to accumulate, is boiled in a vacuum, and 
the gases expelled are collected and introduced into a 
sensitive gold-leaf electroscope. If radium is present 
in the solution, its emanation causes the leaf to lose 
its charge, and the rate at which the discharge occurs 
under defined conditions can be used accurately as 
a measure of the amount of radium present. The 
test is qualitative as well as quantitative, and there 
is no possibility of making a mistake as to the identity 
of the emanation and of the radium from which it is 

The first result of these experiments, while they 
furnished the first evidence of a growth of radium, 
withal in very minute amount, showed that this growth 
is not due to uranium. In the first experiments the 
uranium salt was only specially purified from radium, 
not from any other impurities that might have been 
present, derived from the minerals from which uranium 
is obtained, and a very slow growth of radium from the 
preparation was actually observed. 

In later experiments more perfect methods of purify- 
ing the uranium initially were adopted, with the result 
that the growth now of radium occurred chiefly in the 
impurities separated, whilst the growth in the purified 


radium was reduced to an excessively minute amount. 
In these the greatest growth recorded was only one fifty- 
millionth of a milligram of radium after six years. At 
this rate, even at the present enormous price of radium, 
it would require sixty thousand years to produce one 

Now if uranium X, when it disintegrates, produced 
radium directly, then with the quantities of materials 
used in these later experiments, the amount formed in a 
single hour would be greater than has actually been 
formed in six years. In the earlier experiments, with 
not specially purified uranium, the growth of radium, 
although quite detectable, was still only one thousandth 
part of what would have occurred had uranium X 
changed directly into radium. In spite of this appar- 
ently conclusive negative result, it was practically certain 
that uranium is the original parent of radium, and that 
in the course of years our preparations would begin to 
grow radium. 

Existence of Intermediate Products. 

The natural explanation of this failure to detect a 
growth of radium from uranium is, that one or more 
intermediate bodies of long life exist in the disintegration 
series between uranium and radium. On the analogy 
proposed, this means that between Loch Katrine and 
Milngavie reservoir one or more large intermediate 
reservoirs exist, which have to fill up before the water 
reaches Milngavie. Uranium X represents the first of 
such a series of intermediate reservoirs, it is true, but 
owing to its short period of life and the large fraction of 
the total quantity always passing through on the way to 
the next, such a reservoir would be an extremely small 
one, and for periods such as we are considering its effect 
on the flow would be practically negligible. 

It would be quite otherwise if one or more reservoirs 
as large as Milngavie — if one or more intermediate 
substances as long-lived as radium — existed in the series. 


1 well remember one fact told me by the engineer in 
charge of the magnificent scheme of waterworks, supply- 
ing the mines at Kalgurli, in Western Australia, from 
a source near the coast across three hundred miles of 
desert. There are several intermediate reservoirs on 
the way. The plant installed is capable of pumping 
five million gallons of water daily, and yet it took 
a period of many weeks since pumping operations began 
before the water appeared in Kalgurli. When uranium 
is carefully purified from all other substances one can be 
sure that one starts with all the intermediate reservoirs 
empty — ^that is, with none of the intermediate substances 
present. Water is flowing steadily from the source all 
the time, as the disintegration of uranium is always 
going on. We watched and waited seven years at the 
radium reservoir — strictly speaking, at the one beyond 
radium, since the emanation of radium, not radium 
itself, is actually employed for the test. But the flow 
had not reached there yet and the radium reservoir 
remained practically as empty as at the start. But 
there was no doubt it would come, and there was good 
reason to expect that some of us, at least, would be still 
alive when it arrived. 

It is not beyond the resources of mathematics to 
find out a good deal about these intermediate reservoirs. 
The present results indicate that if there is but one long- 
lived intermediate body between uranium and radium, 
then its period of average life must be at least 100,000 
years, that is, forty times that of radium itself. Also, 
that the radium, in this case, must be produced at a 
rate proportional to the square of the time from purifi- 
cation, the growth in a century being a hundred times 
as great as that in the first decade. On our analogy, 
then, between Loch Katrine and Milngavie, there must 
exist a reservoir of forty times the capacity of Milngavie, 
provided there is only one. Since the equilibrium 
quantity to which an intermediate body accumulates is 
proportional to its period of average life, then if there is 
only one intermediate parent of radium between radium 


and uranium, there must be forty times as much of it 
in minerals containing radium as there is of radium 


This leads me to the next step. The failure to detect 
a production of radium from uranium merely fore- 
shadowed the actual discovery of an intermediate sub- 
stance of long period of life. Boltwood in America suc- 
ceeded in isolating it from minerals containing radium, 
and it proves to be the direct parent of radium. It 
possesses the property of producing radium directly 
from itself by disintegration, and it has been called 
ionium. It expels a-rays during its disintegration into 
radium, and these a-rays possess a relatively low velocity. 
Their range is very little more than one inch of air. 
Chemically, ionium resembles thorium so completely 
that the two substances, if mixed, cannot be separated. 
This gives the means of separating the new body from 
minerals. Some thorium is added and separated by the 
well-known methods of chemical analysis. It is then 
purified as completely as possible. The parent of 
radium is not separated from the thorium by this treat- 
ment, although all other substances are. The chemical 
resemblance between these two different elements is 
complete. Later we shall come to recognise many other 
cases of the same kind. Ionium and thorium are what 
are now called isotopes. 

The disintegration series thus reads: 

^ <0'* y^'^y^ p*" P" P" 

I 238 )-— ^ I 23* )~-^ — — — **^( 2^° j"""^ { ^2^ 1 ■ / I 222 I— — 

Uriniuml. Uranium X. Ionium. Radium. Emanation. 

8,00,000,000 35-5 days. 100,000 years. 2,500 years. 5-6 days. 


Fig. 29. 

as far as we have yet considered it. In the centre is 
placed the known or presumed atomic weights of the 
various bodies. 


Production of Radium by Uranium. 

Going back to the purified uranium preparations, in 
1915 with the help of Miss Hitchins, the measurements 
of the quantity of radium present first clearly estab- 
lished that there was a steady growth of radium, and, 
moreover, that it was proceeding proportionally to the 
square of the time, as the theory requires. This growth 
has continued regularly up to the present time (1919). 
The period of average life of ionium calculated from it is 
almost exactly 100,000 years. The amount of radium 
in some of the preparations is (1919) about ten times as 
great as initially. But the problem has taxed to the 
uttermost even the extraordinarily delicate tests for 
radium. For the preparation containing the largest 
quantity of uranium — namely, three kilograms calcu- 
lated as the element — the growth of radium after ten 
years has been only one five millionth of a milligram — 
i.e., one part of radium from fifteen billion of uranium. 

The Stately Procession of Element Evolution. 

So far, then, as we have inquired, uranium, uranium 
X, ionium, radium, and the emanation represent 
respectively the starting-point and the four successive 
stopping-stations in the long journey of continuous 
devolution from the heaviest and most complex atom 
known into less heavy and complex atoms which is going 
on around us, or, to preserve our original analogy, the 
source and four successive intermediate reservoirs in 
the flow of elementary evolution. " AH things flow " 
was one of the dogmas of ancient philosophy, and in this, 
as in many others, the ancients guessed truer than they 
knew. Instead of four stopping-stations or inter- 
mediate reservoirs in this stately procession of elements 
disclosed by radioactivity, there are now known no 
less than thirteen, starting from the element uranium, 
but for our present purposes of illustration these four 
will suffice. But this new transformation scene on 


which the curtain of the twentieth century has been 
rung up, beginning as it has done with the transforma- 
tion of the most fundamental and permanent of the 
existences which physical science has recognised in 
the past, extends beyond physical science and trans- 
figures with new light some of the most fundamental 
and permanent ideas which in one form or another are 
deep-rooted in the world's philosophies. 



The Later Changes of Radium. 

We have attempted to trace radium to its source. 
It remains to follow through its disintegration briefly 
to the end. This was a task to which Rutherford 
particularly devoted himself, after the main principles 
of atomic disintegration had become famiUar, with the 
consequence that, with the exception of a lacuna here 
and there still to be supplied, our knowledge of the whole 
process from the start to finish is now tolerably complete. 
In addition, some new considerations have transpired 
which concern us nearly in the broad general application 
of the principles of atomic disintegration, so that for this 
reason, if for no other, the work claims our attention. 

Most of you who have read at all in the subject will 
be aware of one mysterious and extraordinary power 
possessed by radium, which I have hitherto carefully 
avoided all mention of, not wanting to have too many 
irons in the fire at once. Radium possesses the power 
of endowing with some of its own radioactivity neigh- 
bouring objects. Thorium, which is very like radium 
in many ways, particularly in giving a gaseous emanation 
(which, however, has the very short period of average 
life of only a little over a minute), also possesses a 
similar power. The phenomenon was discovered by the 
Curies for radium and termed " induced radioactivity," 
and for thorium simultaneously by Rutherford and 
termed " excited radioactivity." With the explanation 
of the property the original names have largely fallen 
into disuse. We shall now confine ourselves to the case 



of radium. Any object left in the immediate neigh- 
bourhood of a radium salt becomes radioactive, but after 
it is removed the radioactivity decays away rapidly 
and almost completely, abnormally at first, but sub- 
sequently more regularly, with a half-value period 
approaching thirty minutes. The temporary activity 
so " induced " consists of a-, ^-, and 7-rays. The 
activity exists as an invisible film or deposit over the 
surface of the object rendered radioactive, for, by sand- 
papering, the activity can be rubbed off and then is 
found on the sand-paper. It is now customary in con- 
sequence to refer to it as the " active deposit of radium." 
This power is, strictly speaking, not a property of 
radium itself, for if the radium is contained in a com- 
pletely closed vessel — it does not matter how thin- walled 
so long as it is air-tight — no radioactivity whatever is 
produced outside. The first step in understanding the 
nature of the phenomenon consisted in tracing it to the 
action of the emanation of radium. In the ordinary 
condition the emanation is always diffusing away to some 
extent from radium salts unless they are contained in 
air-tight vessels. The " active deposit " is the product 
of the disintegration of the emanation. Just as radium 
cannot exist without continuously producing the emana- 
tion, so in turn the emanation cannot exist without 
continuously producing this active deposit. In any 
vessel containing radium emanation this body is being 
continuously deposited on the walls of the vessel, so 
that if the emanation is at any time blown out, the 
active deposit remains behind. Radium expels one 
a-particle and changes into the emanation. The emana- 
tion expels a second a-particle and changes back again 
into a solid, or at least into a non-gaseous form of 
matter, the first of the " active deposit " group. The 
latter in turn expels more a- and also /S-particles, and 
so the course of successive disintegrations goes on. 
In the active deposit itself at least three changes follow 
one another with great rapidity, so that the analysis 
of them proved a complicated task. 


The Active Deposit of Radium. 

You know that if a moisture-laden atmosphere is 
sufficiently chilled, the vapour of water condenses 
directly into the solid form, and a snowstorm results. 
Something of this kind is always happening in an atmo- 
sphere containing the radium emanation. Every second 
two out of every million of the atoms of emanation dis- 
integrate, expelling «-particles and leaving a solid 
residue, so that there is a sort of continuous snowstorm 
silently going on covering every available surface with 
this invisible, unweighable, but intensely radioactive 
deposit. Unlike snow, however, the particles of this 
active deposit are charged with positive electricity, 
so that if two surfaces are provided, one charged nega- 
tively and the other positively, the deposit is attracted 
almost entirely to the negatively charged surface. The 
other surface repels the particles and so does not get 
coated. By making the negatively charged surface 
very small the active deposit can be almost entirely con- 
centrated upon it. This enables me to show you more 
effectively the production of the active deposit from the 
emanation and some of its chief properties. The 
separation of the non-volatile product of a volatile 
parent or emanation by this use of a negatively charged 
surface is a very simple operation, much more so than 
when the parent substance is non- volatile and the recoil 
of the product is used to effect its separation and con- 
centration on a negatively charged surface, as discussed 
on p. 104. 

It would take us too long and too far if we attempted 
first to study these properties, and then tried from them 
to deduce their explanation. It must suffice if I give 
you first the explanation of the facts according to the 
theory of atomic disintegration and then illustrate as 
many of the points in it as possible experimentally. 
I have said that after the disintegration of the emana- 
tion at least three successive disintegrations, following 
one another rapidly, occur. The bodies produced are 


referred to as radium A, radium B, radium C, in 
order to avoid the necessity of inventing a host of new 
names for bodies having such fleeting existence (Fig. 30), 

Radium. Emanation. Radium A. Radium B. Radium C. 
"■ » '' 

Active deposit of rapid change. 
2,500 years. 5"6 days. 4-3 minutes. 38 -5 minutes. 28 •! minutes. 

Fig. 30. 

As before, the presumed atomic weights are placed 
inside the circles corresponding with the successive 
products. The periods of average life are placed below. 
The symbol (/3) here - and throughout indicates that 
/S-rays are expelled, but that they are not the normal 
penetrating ^S-rays, but rays akin to the cathode-rays 
in their low penetrating power and low velocity. They 
only come into evidence in special experiments, and are 
not of great general importance. The first body pro- 
duced from the emanation, radium A, changes with great 
rapidity with a period of average life of 4-3 minutes, 
expelling an «-particle. The body radium B resulting 
undergoes a change which was at first thought to be 
entirely " rayless." Neither a- nor /9-rays of the ordin- 
ary kind can be detected, although a very feebly pene- 
trating jS-ray is produced, which we need not further 
consider. The period of this substance is 38-5 minutes. 
The body produced, radium C, changes, expelling both 
a- and /3-particles and 7-rays also. The period is 28-1 
minutes. It is probable that this change is complex 
and that the /3- and 7-rays are given off in a separate 
change to that in which the a-rays result. This point 
will be dealt with later. 

The Radiations from the Active Deposit. 

We started our description of the rays of radium 
with the statement that they consisted of a-, yS-, and 


7-rays. One of the most interesting points of the above 
scheme is to show that the /9- and 7-rays do not come 
from radium itself, any more than they do from uranium 
itself, but from the later products. It is loose, but con- 
venient, to talk of the /3- and 7-rays of radium. Really 
we mean the /3- and 7-rays of radium C. The emanation, 
like radium itself, gives only a-rays. The whole of the 
/3-rays result in the later changes of the active deposit. 
We have seen that, freshly prepared from solution, 
radium salts give only a-rays. The ^- and 7-rays 
make their appearance only after the subsequent pro- 
ducts have accumulated. 

Experiments with the Active Deposit. 

On the table there is a small glass vessel silvered 
internally (Figs. 31 and 32) containing the emanation 
from half a grain of radium bromide. It is arranged 
so that steel knitting-needles can be inserted into the 
emanation and withdrawn through a glass tube held 
in a cork. The needle is connected to the negative pole 
of the electric supply and the silver coating to the posi- 
tive pole. If only the point of the needle is made to 
project beyond the glass tube, the whole of the active 
deposit can be concentrated on the point. Some hours 
before this lecture a needle — we will call it No. 1 — was 
so inserted, and by now its point should be coated to 
its maximum degree of radioactivity with the products 
of the disintegration of the emanation. After some 
hours the products all arrive at the state of radioactive 
equilibrium, in which the quantity is at its maximum for 
all the products, radium A, radium B, and radium C, 
as much of each changing as is produced from the emana- 
tion. The disintegrations all going on together, the 
wire should give a-, /S-, and 7-rays, the /3- and 7-rays 
being as intense as those given from the half-grain of 
radium bromide from which the emanation was derived. 
Now I withdraw No. 1 needle from the emanation, and 
with the room darkened we will examine its active 

Fig. 32. — Apparatus for obtaining the Active 
Deposit of Radium. 

To face p. 141 


To detect the a-rays we will use a glass translucent 
screen, thinly coated with phosphorescent zinc sulphide 
on one side. I bring the point of the needle gradually 
near the coated side of the screen. As soon as it comes 
within a distance of three inches the screen lights up, 
and when the point is only a little distance removed 
from the screen a most brilliant phosphorescence is 
produced. Now if I interpose between the wire and 
the screen a single sheet of paper, the effect practically 

Fig. 31. 

entirely ceases. The a-radiations producing this effect 
come both from radium A and from radium C. 

To detect the y9-rays we will use an ordinary card- 
board X-ray screen of barium platinocyanide. Bring- 
ing the needle behind the screen, so that the rays 
have to penetrate the cardboard, you observe the screen 
lights up as brightly as with half a grain of radium 
bromide itself. In the dark I happened actually to 
touch the back of the screen with the active needle- 
point, and in so doing some of the active deposit has 



been transferred to the back of the screen. You can 
see where the back of the screen was touched, because 
this spot still glows though the needle has been removed. 

If now the needle is again presented to the back of 
the X-ray screen with thin pieces of metal foil inter- 
posed, you see that the rays are only slightly stopped 
by having to traverse the foil. When a piece of thick 
lead sheet is interposed, a faint luminosity on the screen 
still remains produced by the 7-rays. In fact the active 
needle-point gives all the penetrating rays given by half 
a grain of radium bromide. 

It is now several minutes since the needle was removed 
from the emanation. If we now again examine the 
a-rays you will notice they already are very perceptibly 
less intense than at first. Practically all the radium A, 
of which the period of average life is only 4-3 minutes, 
has already disintegrated, and in consequence the a-rays 
now come only from the radium C, and are only half as 
intense as at firsts 

Radium A. 

Now if, instead of exposing the needle to the emana- 
tion for some hours so as to allow all the successive 
products time to be produced, we expose it to the emana- 
tion for a very short time, say for five minutes by the 
watch, we shall get quite a different set of effects. Here 
is a new needle; we will call it No. 2. Before putting 
it in I will test it with the screen to show you that at 
present it is an ordinary needle, not at all radioactive. 
We will let it stay in the emanation, connected to the 
negative pole as before, for five minutes and withdraw 
it, and test its a-rays immediately, exactly as before. 
You observe that it is already giving a-rays abundantly. 
Comparing it with No. 1, the two are now very similar 
in their a-ray-giving power, No. 1 being only slightly 
the better. The a-rays from No. 2 come almost entirely 
from radium A, for there has not yet been time for any 
appreciable quantity of radium C to be formed. The 
a-rays from No. 1 come entirely from radium C, and this 


radiation has not yet had time appreciably to decay. 
Let us, however, test their yS-rays. You observe that 
No. 2 gives no yS-rays worth considering, whereas No. 1 
still gives y8-rays in practically undiminished intensity. 
Radium A gives no ^S-rays, and as there is no appreciable 
quantity of radium C formed there yet, the consequence 
is that No. 2 wire gives no /3-rays. 

I can show you at this stage a very striking experi- 
ment with another needle, No. 3, which has been in the 
emanation a few minutes. I take it out and draw the 
point once through a piece of emery-cloth and expose 
the latter to the zinc sulphide screen. You observe that 
a single rub has removed a large part of the active de- 
posit from the needle and transferred it to the emery- 
cloth, so that the latter makes the screen glow almost 
as brilliantly as the needles themselves. 

Radium B and C. 

Now we will contrast the decay of the activity of the 
needles Nos. 1 and 2. The activity due to radium A 
by itself decays very rapidly, half disappearing every 
three minutes. The consequence is, if we now again 
test the a-rays of No. 2, we shall find they have already 
nearly disappeared, whereas No. 1 still continues to 
give a-rays at about the same strength as it did when 
last examined. In ten minutes the «-rays of No. 2 
practically disappear. 

It is thus not difficult to give you a certain amount 
of experimental evidence in favour of the conclusion 
that the first change of the active deposit is a very lapid 
one in which a-, but no yS-rays are expelled, and that 
this is followed by a less rapid change in which both 
a- and yS-rays are expelled. It is more difficult to give 
you in a lecture satisfactory evidence of the existence 
of radium B, a body not itself giving rays, intermediate 
between the first and second changes in which rays are 
expelled. If we examine carefully the decay of th e a- 
and ^-rays of wire No. 1, in which at first all these pro- 


ducts co-existed in equilibrium, we shall find, as already 
shown, that for the first half-hour after removal from 
the emanation the /S-rays suffer very little change and 
then the regular decay begins. In the next half-hour 
the /9-rays decay approximately to one-half their original 
intensity, and the decay then goes on at this rate regu- 
larly and continuously to the end. After two hours 
they are only a few per cent, of what they originally 
were, and in three or four hours they can no longer be 
detected. The initial pause before decay begins is due 
to the quantity of radium C being maintained, in spite 
of the fact that it is disintegrating all the time, expelling 
a- and )S-rays, by the disintegration of radium B. The 
latter continues to supply new radium C to replace that 
disappearing for the first half-hour or so after the 
needle is removed from the emanation. Exactly the 
same pause occurs in the decay of the «-rays. As we 
saw with No. 1, within a very few minutes after the 
needle was removed from the emanation the a-rays 
had decayed to about one-half, owing to the disappear- 
ance of the a-ray-giving radium A. Then, however, 
little further change occurred. It is now about half 
an hour since No. 1 was first tested, and the a-activity is 
similar to what it was when last tested twenty minutes 
ago. The a-rays of No. 2 have now almost completely 
disappeared. If we continued to examine No. 1, we 
should find, from now on, a rapid decay of both a- and 
/3-rays at the same rate, so that at the end of the lecture 
both will be much enfeebled, and by midnight both 
will have ceased so far as we could tell by these rough 

The Radiation from the Emanation. 

Now that we have finished with the emanation 
used in the preceding experiments, it is an interesting 
experiment to show that itself it gives no ^-rays. If 
we blow the emanation out into a U-tube of thin glass 
cooled in liquid air, it is condensed in the cold tube. 


The tube can then be sealed up to prevent the emanation 
from escaping. The tube contains some phosphorescent 
zinc sulphide and glows brightly owing to the a-rays 
from the emanation inside. But if we hold the tube 
against the X-ray screen, you can see that no penetrating 
rays come from the tube. The emanation itself gives 
no /3-rays, only a-rays. By the end of the lecture, how- 
ever, sufficient radium C will probably have been 
formed inside the tube to give an appreciable /S-radia- 
tion. Owing to the existence of the intermediate body 
radium B, there occurs a similar pause in the growth of 
/3-rays from the emanation to that which, as we have 
seen, occurs in their decay, after the emanation is taken 
away. But in two or three hours the /S-rays from all 
the needles will have decayed, and that from the sealed 
U-tube will have reached a maximum. 

The Later Slow Changes of Radium — 
Radium D, E, and F. 

This finishes this subject and brings us to the next. 
What happens to radium C when it disintegrates ? Is 
this the real or only the apparent end of the process ? 
It is, in fact, a very long way from the end. Madame 
Curie discovered that the rapid and almost complete 
decay of the active deposit, at the end of a few hours 
after removal from the emanation, is not in fact quite 
complete. A very small residual radioactivity remains 
and persists for years. The series of changes have now 
entered on a stage which is as slow as the previous ones 
were rapid. The next change requires almost as many 
years as the last required minutes for completion. 
The effect of these further changes is in consequence 
extremely small, but they last a very long time. Con- 
tinuing our diagram where it last ended at radium C, 
the next stage is represented in Fig. 33. 

The body produced from radium C, radium D, has 
a period of many years. It is too early yet to state it 
exactly. One recent estimate makes it twenty-four 


years. No very important rays are given in its change. 
/?-rays, however, result from the body produced from it, 
which changes rapidly again with a period of only a 


Radium C. Endium D. Radium E. Radium F. Radium Q. 

(Polonium.) (Lead?) 

« . ' 

Active deposit of slow change, 
28*1 minutes. 24 years (?) 7-5 days. 202 days. 

Fig. 33. 

few days. We shall pass over these intermediate changes 
and consider the last known change of the series, that 
of radium F, which has a period of average life of 
202 days, in which an a-particle is expelled. Radium F 
is the 'polonium of Madame Curie, having been separated 
by her from pitchblende first before she discovered 


A digression may here conveniently be made on what 
is known about polonium, before its connection with 
radium is considered. Chemically it resembles bismuth, 
and was separated first from pitchblende in association 
with the bismuth contained in the mineral. Its radio- 
activity, which consists entirely of a-rays, slowly and 
completely decays, so that a few years after it has been 
prepared, the most intensely active preparations of it 
lose practically all their activity. The work was carried 
on by Marckwald in Germany, who discovered new and 
simple methods of extracting polonium from the mineral 
and worked up many tons of pitchblende for this sub- 
stance. His careful chemical investigations of the 
nature of the body made it clear that it was quite as 
nearly allied in chemical nature to the element tellurium 
as to bismuth, and he first proposed the name " radio- 
tellurium " for it, which, however, with the elucidation 


of its identity with polonium, has fallen into disuse. 
He proved that there is far less polonium in the mineral 
even than radium. In a ton of mineral there is less than 
a thousandth part of a grain of polonium, but the radio- 
activity is correspondingly intense, and greatly exceeds, 
so far as the a-radiation is concerned, that of pure radium 
itself. The period of average life, 202 days, is deduced 
by direct observation from the rate of decay of the radio- 

Returning now to the consideration of radium C, 
we saw that after its activity had decayed there existed 
still a residual activity which is very feeble. This 
steadily increases with time, and consists both of a- and 
/3-rays, which, however, increase at different rates. The 
a-rays are due to polonium, or radium F. These go 
on increasing for the first two years and then a maximum 
is reached, the amount of the radium F formed being in 
equilibrium. The /3-rays, however, reach a maximum 
much more quickly. The /3-ray product (radium E) 
having a much shorter period, equilibrium is reached 
in a few weeks. If at any time the active matter is 
subjected to the chemical processes worked out by 
Marckwald for the separation of polonium, the a-ray 
body radium F can be separated from the other products, 
and its activity then decays away completely at exactly 
the same rate as in the case of polonium. Moreover, 
it shows the property of being volatile at a temperature 
of a bright red heat, which is the basis of one of the 
methods originally used by Madame Curie in separating 
polonium from the bismuth in pitchblende. This is 
merely a sketch of the evidence in favour of regarding 
polonium as the last radioactive substance produced 
in the disintegration of uranium. 

The Ultimate Product of Radium. 

One more step remains to be discussed, and then 
this long story of continuous transformation is at an end. 
What is the ultimate product ? When radium F or 


polonium expels its a-particle, what is produced ? 
The estimated atomic weight of polonium is 210, which 
is deduced by subtracting from the atomic weight of 
radium (226) the weight of the four atoms of helium 
known to be expelled in the form of a-particles. This 
agrees well with its chemical nature, for there is a 
vacant place in the periodic table for an element, the 
next heavier than bismuth (atomic weight, 208-5), and 
this element would be chemically analogous to tellurium. 
The expulsion of an a-particle would further reduce the 
atomic weight four units, leaving a residue of atomic 
weight 206. What is it ? 

Now, if this is really the final product and not merely 
a very sloAvly changing substance, the formation of 
which in proportion to the degree of slowness of the 
change would be difficult experimentally to detect, then 
it follows that the ultimate product must accumulate in 
quantity indefinitely with time in the minerals contain- 
ing the elements of the uranium-radium series, and must 
therefore be a well-known common element. Lead has 
the atomic weight of 207-2, and bismuth, 208-0. The 
next known element is thallium (204), and then comes 
mercury (200). 

Lead is found in all the common minerals containing 
uranium in considerable quantity, and there is also 
evidence that the older the geological formation from 
which the mineral is obtained, the greater the percentage 
of lead present. Recently a uranium mineral, autunite, 
has been found containing no chemically detectable 
quantity of lead. But then the same mineral contains 
only an excessively minute trace of helium, and less 
than its full equilibrium amount of radium. There is 
every reason to believe that its formation as a mineral 
has occurred in quite recent times. 

This question has now been settled by indirect means, 
and there is no longer room for doubt that lead is the 
ultimate product of uranium. This evidence, however, 
may be deferred. The method of settling it directly is 
to study the change of polonium, separated from enor- 


mous quantities of pitchblende, by the aid of the spec- 
troscope, and on this task Mme. Curie and her colleagues 
have for long been engaged, but as yet without definite 
proof that lead is the product. 

Uranium I and Utjanium II. 

A variety of evidence, some of which may be dealt 
with more profitably later, has lately established the 
conclusion that the change suffered by the uranium 
atoms when the a-particles are expelled, is not, as first 
supposed, a single change. The substance uranium, 
which chemists have hitherto considered an element, 
differs from every other known substance expelling 
«-rays, in that, per atom disintegrating, two a-particles 
are expelled instead of one. Moreover, these two 
a-particles are 'expelled at slightly different initial 
velocities, with the result that the " ranges " of the 
two sets of a-rays in air are slightly different (see p. 164). 
Most probably the two a-particles are not expelled from 
the uranium atom simultaneously but successively. In 
consequence, what chemists hitherto have accepted as 
a single element is, in reality, a mixture of two, chemi- 
cally so much alike that they have not yet been separ- 
ated, the first having the atomic weight 238-5, and which 
has been termed provisionally uranium I; the second, 
resulting after the expulsion of the first a-particle, having 
the atomic weight 234*5. It has been termed uranium 
II. It is probable that this uranium II is present in 
relatively very insignificant proportion by weight, 
although it contributes one-half of the total a-radiation. 
Its period of life can only be estimated from very indirect 
and incomplete data at the present time. The more 
slowly a radioactive substance changes the shorter the 
range of the a-particles it expels, and so from the range 
of the rays an estimate of the period of average life 
may be formed. This estimate, such as it is, attri- 
butes a period to uranium II of about two million 


Numerous similar examples of elements identical 
chemically, but differing in radioactivity, are now 
known. These are called isotopes or isotopic elements. 

Uranium X and Uranium X, (Brevium). 

Even more recently it was first predicted and then 
shown that uranium X is not a single substance. Ura- 
nium X gives two kinds of /S-rays, one of low velocity 
and comparatively non-penetrating — i.e., (13-) rays 
(compare p. 139) and ordinary high velocity /3-rays. 

® ^'^^^ •PC&y) @ @ © 

(238) >(234) ^^34) ^(234J ^(230) ^(226) > 

Uranium I Uranium X^ Uranium X 2 Uranium II Ionium Radium 

8,000,000,000 35.5 days 1.65 minutes 3,000,000 100,000 2,500 years 

years years (?) years 

^22) ^(218 j >- r214j >- (214.") ^ M214J > 

Emanation ,RadiumA Radium B Radium C Radium C^ 

'" ^ Active Deposit of Rapid Change 

4-3mins. 38-smins. 28-1 mins. One millionth 

,^ , , . ,^ of a second (?) 

^lOj ^(210j >- (210) ^ (2O6) 

Radium D Radium E Radium F Radium G 

^ (Polonium) (Lead) 

Active Deposit of Slow Change 
24 years 7-25 days 202 days 

Fig. 34. 

These have been shown to originate from two distinct 
substances successively produced, and which are called 
uranium Xj and uranium Xg. Uranium I expels an 
«-particle and changes into uranium Xj, which has a 
period of average life of 35-5 days, and expels, not the 
penetrating /S-rays of " uranium X," but the feeble and 
unimportant (/8)-radiation. Its product, uranium Xg, 
sometimes called brevium, has the very short period of 
average life of only 100 seconds, and, in its change, the 
powerful penetrating /3-rays are expelled. Uranium 
Xj, after its change, is believed to become uranium II. 


The latter in its change expels an a-particle and is 
believed to produce ionium. Uranium Xj and uranium 
Xo, unlike uranium I and uranium II, can be separated 
from one another by chemical methods. These new 
discoveries, although of highest theoretical importance, 
make very little practical difference to the results, which 
for almost all ordinary purposes are precisely what they 
would be were the simple scheme, shown in Fig. 29, 
actually the one followed. 

Radium C and Radium C. 

Lastly, there is indirect evidence that radium C 
consists of two successive products, distinguished as 
radium C and radium -C, the first giving the ^- and 
7-rays in its disintegration, and producing the second, 
which has a period of average life of only a millionth of 
a second, and changes, emitting an a-particle, into 
radium D (see p. 202). 

Fig. 34 shows so far as it is at present known the 
complete disintegration series of uranium. 



Ratio of Quantities of Polonium and Radium 
IN Minerals. 

From the law, which has already been found so useful, 
we can calculate the ratio of the quantities of radium 
and polonium that exist together in a mineral from 
their periods of average life. The period of average life 
of radium is 4,500 times that of polonium, so that there 
must be 4,500 times more radium than polonium in 
minerals. A good pitchblende with 50 or 60 per cent, 
of uranium in it contains about an ounce of radium 
in 150 tons. The same quantity of polonium would 
therefore be contained in about 700,000 tons. The 
whole output of the Joachimsthal mine per annum, 
reckoned as 15 tons, contains about one hundredth of 
a grain of polonium. This is borne out by Marckwald's 
experiments, already referred to. 

Let us apply the law not only to radium and polo- 
nium, but to the whole list of known transition-forms 
existing as products of uranium. In the table this has 
been done. The first column gives the name of the 
substance, the second its period of average life, and the 
third its relative quantity in minerals, the quantity 
of uranium being considered 1,000,000,000. If these 
numbers are taken to refer throughout to milli- 
grams (1 milligram is about yV of a grain), then since 
1,000,000,000 milligrams is roughly a ton, the quan- 
tities refer to an amount of mineral containing one ton 
of the element uranium. 





Uranium I, 8,000,000,000 years 
Uranium X .. 35-5 days. 
Uranium X2 

Uranium II, 3 
Radium A 
Radium B 
Radium C 
Radium D 
Radium E 
Radium F 

1-6 minutes, 
,000,000 years (?). 
100,000 years. 
2,500 years. 

5-6 days. 

4-3 minutes. 
38'5 minutes. 
28'1 minutes. 
24 years (?). 
7-5 days. 
202 days. 


1,000,000,000 mg. (=1 ton). 

One eightieth mg. 

l/250,000th mg. 

400 grams (?). 

12-5 grams. 

312-5 mg. 

One five-hundredth mg. 

One millionth mg. 

Nine millionths mg. 

Seven millionths mg. 

3 mg. (?). 

One four-thousandth mg. 

One fourteenth mg. 

These respective quantities in the last column emit 
a similar number of «-particles per second in the eight 
cases where a-particles are expelled at all, and so pro- 
duce similar radioactive effects. This is an illustration 
of the compensating principle I spoke of earlier, that 
the quantity of a radioactive substance divided by its 
life, not the quantity only, gives a measure of its radio- 
active effects. It can readily be calculated that the 
actual amount of radium A used in our experiments, 
which produced powerful and striking effects on the 
phosphorescent screen, was much below one ten- 
milHonth of a milligram, or below one thousand- 
millionth of a grain. For it was derived from 30 mg. — 
i.e., half a grain of radium bromide. Yet while it lasts 
it comes into evidence through the energy of the a- 
particles expelled in its rapid disintegration no less 
than any of the other products. 

Impossibility of Concentrating Many of the 
Products of Disintegration. 

The table brings out clearly that radium is but one of 
many radioactive substances in uranium minerals, 
which would be of value if they could be extracted. 
Uranium II, ionium and radium D, all possess sufii- 


ciently extended periods of life to repay recovery. 
Ionium gives only very feebly penetrating a-rays, and 
so would not be so generally useful as radium, whereas 
uranium II and radium D both, being followed by short- 
lived products which give ^-rays, would be of great 
general utility. The reason which has precluded the 
practical separation of these substances in the past is a 
general one, which has proved to be of the highest 
philosophical significance in the chemistry of these new 
ephemeral elements. They all so closely resemble one 
or other of the known elements that the separation is 
impossible. The resemblance between radium and 
barium is of great practical utility, because these two 
elements, though very closely alike in chemical nature, 
can be separated from each other after they have first 
been separated from every other element. Taking them 
in order, uranium II cannot yet be separated from 
uranium I, ionium cannot be separated from thorium, 
nor radium D from lead. Lead, as has been stated, is 
almost always present in considerable quantity in ura- 
nium minerals, and so usually is thorium, but to a much 
more variable extent. Hence, though it is easy to 
separate radium D from the mineral with the lead, it is 
at present useless practically, as it cannot be concen- 
trated from the lead. By choosing suitable minerals 
like secondary pitchblendes, which do not contain 
ponderable quantities of thorium, intensely active pre- 
parations of ionium can however be separated. It is 
at present the only one in the uranium series likely to 
become useful, and its lack of penetrating rays is a 
serious drawback. Polonium, with its period of less 
than a year and its absence of penetrating rays, hardly 
repays extraction, except for purely scientific investiga- 
tions. There is, however, another disintegration series, 
that of thorium, which offers a better chance of 
providing an efficient substitute for radium, and this 
series will therefore be briefly considered in a later 


Increase of Radioactivity of Radium with 

The increase of the radioactivity of radium after it is 
prepared is due to the steady growth of the products 
undergoing further disintegration. As we know, when 
freshly prepared from solution, the activity of radium 
is due solely to its own disintegration and consists of 
a-rays. After four weeks the first four products accu- 
mulate to their equilibrium, and the activity now 
consists of a-, yS-, and 7-rays, the a-rays being four 
times as great as initially. It is not difficult to see that 
the later slow changes must also cause a very slow 
further continuous increase of all these types of rays, 
due to the growth of radium E and polonium from 
radium D. These considerations are embodied in the 
following table giving an analysis of the total radio- 
activity of a radium preparation, kept in a sealed vessel 
so that none of the products escape, at different periods 
since preparation: 



I. Freshly prepared. 

1 (due to radium itself) 

II. After one month. 

4 (1 due to radium) 

1 or 2 

(1 due to emanation) 

(due to Ra C) 

(1 due to radium A) 

(1 due to radimn C) 

II. After a century. 

5 (as in II and 1 due 

2 or 3 

to radium F) 

(1 due to Ra Eg) 

The Rarity of Elements. 

The idea, which is a necessary consequence of the 
atomic disintegration theory, that fixed definite relation- 
ships must exist between the quantities of elements 
formed from one another — for example, between ura- 
nium, radium, and polonium — forms the first indication 
that physical laws may exist regulating the relative 
abundance and scarcity of elements in Nature. Gold 
and platinum, for example, are valuable or rare metals, 
and we do not know why. Radioactive bodies like 


radium are rare because of the rapidity with which they 
are changing. The degree of radioactivity of an element 
being proportional to the rate at which it is changing, it 
follows that radioactive elements are scarce and valuable 
in proportion to their radioactivity. In this case degree 
of radioactivity is a physical measure of value or rarity. 
It is, for example, so far as we can see, an impossibiUty 
that an element like radium will ever be found in greater 
abundance in any minerals than in those already known. 
Naturally, in the consideration of some of these ques- 
tions of general interest upon which we are now entering, 
we are, be it said, in sharp contrast to almost everything 
we have dealt with in the subject up to now, frankly 
speculating. But it is helpful and legitimate to specu- 
late upon how far, if at all, the process of atomic disin- 
tegration, discovered for the radio-elements, applies to 
the case of elements not radioactive, of which there is 
as yet no positive evidence that they are changing at 
all. The workers in radioactivity have within their 
province explored thoroughly the process of atomic 
disintegration. They have made clear the laws it 
follows, they have measured the rates at which it occurs, 
and they have established what may be termed its in- 
evitableness or independence from all known influences. 
But there is no reason why the process should be limited 
in its scope to the somewhat special phenomena which 
led to its discovery. 

The Cukrency Metals. 

It is, for example, natural to inquire whether the 
scarcity of elements like gold is fixed by the operation 
of similar physical laws to those which regulate the 
rarity of radium. The race has giown used from the 
earliest times to the idea that gold is a metal possessing 
a certain fixed degree of value, enabling it to be used 
safely for the purposes of currency and exchange. It is 
no exaggeration to say that the whole social machinery of 
the Western world would be dislocated if gold altered 


violently in its degree of rarity — if, for example, in some 
hitherto unpenetrated fastness of the globe a mountain 
of gold came to be discovered.^ Is there not at least a 
strong presumption that this is really as contrary to the 
operation of natural law as the discovery of a mountain 
of pure radium would be ? 

It may, I think, be taken for granted that an element 
changing more rapidly than uranium, for example — 
that is, with a period of average life of less than 
8,000,000,000 years — is not likely to be much more 
plentiful in nature than uranium, and therefore that all 
the common elements — lead, copper, iron, oxygen, 
silicon, etc., etc. — have periods of average life of many 
thousands of millions of years. So far, the traditional 
view that the elements' are permanent and unchanging 
is substantially correct. At the same time, we cannot 
but recognise that inevitably the effects of atomic dis- 
integration, too slow to be otherwise detectable, would 
result in the accumulation of the more stable and 
longest-lived elements at the expense of the others, 
resulting in some sort of equilibrium in which the 
relative abundance of the elements was proportional to 
their respective periods of average life. For example, 
the ratio between the relative abundance of gold and 
silver is roughly but pretty certainly known, owing to 
these metals being employed for currency purposes 
from the earliest times. It is at least a possible view 
to take that the elements gold and silver belong to the 
same disintegration series, both changing very slowly, 
but the gold many times more rapidly than the silver. 
Obviously we are only at the beginning. But already 
it cannot be gainsaid that the interest and importance 
of this process of atomic disintegration is not confined 
to radioactivity only or even to physical science. It 
extends into almost every region of thought. 

1 Since thesf' words were first written the whole social machinery of 
the Western World has been dislocated by violent alterations in the 
purchasing power of gold, and it has been shown to be no longer a safe 
medium f^r currency. (Compare A Fraudulent Standard, A. T. Kitson. 
London: P. S. King and S.n, Ltd., 1917.) 



The Nature of Atoms. 

I now propose considering briefly another question 
of general philosophical interest in connection with the 
recent advances of physical science. Naturally the dis- 
coveries in radioactivity have not been made without 
influencing considerably our ideas on the ultimate nature 
of atoms. In some points older conceptions have had 
to be modified, while in others these conceptions have 
been strangely confirmed. It has always been a matter 
for remark, considering the myriads of individual atoms 
which go to make up the smallest perceptible quantity 
of matter, that there are so few different kinds. The 
number of atoms which go to make up this world, for 
example, would run into at least fifty-four figures, yet 
among them all there are less than a hundred different 
varieties. Moreover, it has come to be regarded as one 
of the greatest philosophical generalisations of physical 
science that all the atoms of one kind, that is to say of 
one element, are, at least as far as was known up to the 
beginning of the present century, completely similar in 
character. There is, for example, not the shadow of 
distinction between gold found in the Klondyke, in 
Australia, or in S. Africa. Not only so, but we have 
learned from the spectroscope that this similarity of 
nature extends throughout the whole universe. In this 
connection, both to set forth the idea and to illustrate 
the deductions which have been drawn from it, I cannot 
do better than to quote a celebrated utterance of Clerk 
Maxwell to the British Association in 1873. I may 
remark that Clerk Maxwell throughout used the word 
molecule in the sense of " atom " as this word is em- 
ployed by the chemist, and throughout these lectures. 

" In the heavens we discover by their light, and by 
their light alone, stars so far distant from each other 
that no material thing can ever have passed from one 
to another; and yet this light, which is to us the sole 
evidence of the existence of these distant worlds, tells 


us also that each of them is built up of molecules of the 
same kinds as those which we find on earth. A molecule 
of hydrogen, for example, whether in Sirius or in Arc- 
turus, executes its vibrations in precisely the same time. 

" Each molecule therefore throughout the universe 
bears impressed upon it the stamp of a metric system 
as distinctly as does the metre of the Archives at Paris, 
or the double royal cubit of the temple of Karnac. 

" No theory of evolution can be formed to account 
for the similarity of molecules, for evolution necessarily 
implies continuous change, and the molecule is incapable 
of growth or decay, of generation or destruction. 

" None of the processes of Nature, since the time when 
Nature began, have produced the slightest difference in 
the properties of any molecule. We are therefore unable 
to ascribe either the existence of the molecules or the 
identity of their properties to any of the causes which 
we call natural. 

" On the other hand, the exact equality of each mole- 
cule to all the others of the same kind gives it, as Sir 
John Herschel has well said, the essential character of a 
manufactured article, and precludes the idea of its being 
eternal and self-existent. 

" Thus we have been led, along a strictly scientific 
path, very near to the point at which science must stop; 
not that science is debarred from studying the internal 
mechanism of a molecule which she cannot take to 
pieces, any more than from investigating an organism 
which she cannot put together. But in tracing back 
the history of matter. Science is arrested when she 
assures herself, on the one hand, that the molecule has 
been made, and on the other, that it has not been made 
by any of the processes we call natural. 

" Science is incompetent to reason upon ^^he creation 
of matter itself out of nothing. We have reached the 
utmost limits of our thinking faculties when we have 
admitted that because matter cannot be eternal and 
self-existent it must have been created." 

You will admit that, in the light of all that has trans- 


pired in the forty-five years since Maxwell used these 
words, science has advanced far. The concluding words 
of the address are even more striking from this point of 

" Natural causes, as we know, are at work, which tend 
to modify, if they do not at length destroy, all the 
arrangements and dimensions of the earth and the whole 
solar system. But though in the course of ages catas- 
trophes have occurred and may yet occur in the heavens, 
though ancient systems may be dissolved and new 
systems evolved out of their ruins, the molecules out of 
which these systems are built — the foundation-stones 
of the material universe — remain unbroken and un- 

Before we dwell upon the modifications that have 
been made in this point of view, let us rather consider 
the chief basis of the argument, namely, that all the 
atoms of any one element are exactly alike. On this 
fundamental question the evidence to-day is far more 
complete and definite than it was in 1873. Recent 
developments in connection with isotopes have modified 
our point of view, but for the moment we may neglect 
this special advance. 

We no longer regard the atom as a simple thing. On 
the contrary, we now look upon it as an almost infinitely 
complex piece of mechanism. The late Professor Row- 
land, of Baltimore, once made the remark that a grand 
piano must be a very simple piece of mechanism com- 
pared with an atom of iron. For in the spectrum of 
iron there is an almost innumerable wealth of separate 
bright lines, each one of which corresponds to a sharp 
definite period of vibration of the iron atom. Instead 
of the hundred-odd sound vibrations which a grand piano 
can emit, the single iron atom appears to emit many 
thousands of definite light vibrations. Two pianos 
would be regarded as in perfect tune together when there 
was a comparatively rough approximation of period 
between the various notes. Whereas by the spectro- 
scope a difference in " tune " or period in the vibra- 


tions emitted by different atoms of only one part 
in many millions would be easily detectable, and no 
such variation exists. In a similar vein Professor 
Schuster, referring to the broad teachings of the spectro- 
scope, has compared the atoms of the same element to 
an innumerable number of clocks all wound and regu- 
lated to go at the same period. If all these clocks were 
set at the same time, not one of them would vary by a 
single second even after many many days. No clock- 
maker could make such clocks. Yet these almost 
infinitely complicated pieces of mechanism we call 
atoms are turned out by Nature with such undeviating 
accuracy and fidelity that in all the myriads in existence 
there are less than a hundred different kinds known. 

The Velocity of a-PARTiCLES. 

We can, however, from the point of view of recent 
researches in radioactivity, push this idea even one step 
further, to the case of atoms actually in the condition 
of breaking up. We have seen that it is a property of 
the a-rays to possess a very sharp and definite range. 
In a beam of homogeneous «-rays passing through a 
homogeneous absorbing medium the number of a-par- 
ticles suffers little or no diminution until the extreme end 
of the path is reached, and then they cease altogether. 
Just without the extreme range, there is absolutely no 
effect perceptible, while just within this range, the effect, 
per small element of path, is at the maximum. Every 
«-particle expelled from the radio-element in the same 
change travels exactly the same distance before it ceases 
to be detectable, and, as Rutherford has shown by direct 
measurement of the magnetic and electric deviation, is 
expelled at the same velocity. 

In the table following, the approximate initial velor 
cities of the a-particles from the changes in the uranium 
series have been collected, together with their " ranges " 
or distances in millimetres they will penetrate in air at 
15° C. and 760 mm. of mercury pressure. 






(miles per second 

i Range 

Uranium I, 

. 8,000,000,000 years 


.. 25 

Uranium II, 

3,000,000 years 


. 29 


100,000 years 


. 30 


2,500 years 


.. 33 


5-6 days 


. 42 

Radiium A 

4-3 minutes 


. 47-5 

Radium C' 

. 1/1, 000 ,000th sec. 



. 69-5 

Radium F 

202 days 


. 37-7 

The atom thus retains its role of a perfect piece of 
mechanism even up to and during the moment of its 
dissolution. So exactly alike are all the atoms of the 
same radioactive element, that when the break-up occurs 
the velocity with which the fragments of the atom, or 
a-particles, are expelled is exactly the same in each case. 
We may liken the disintegration of an element to the 
bursting of shells, in which the fragments of the different 
shells all are expelled with the same velocity. Certainly 
no shells ever constructed would answer this require- 
ment. Truly, in the words of Sir John Herschel, 
the atom bears the essential character of a manufac- 
tured article, but of a degree of perfection humanly un- 

But with regard to the process of manufacture and of 
the cause of this undeviating fidelity to a few types, 
what a revolution of thought has taken place in the last 
few years ! The evolution, or rather devolution, of 
matter, its continuous change, the generation and 
destruction of atoms — all of the things which seemed 
impossible in Clerk Maxwell's day — we know to be 
going on before our eyes. It is true the processes call 
for periods of time so vast, even in the most favour- 
able cases, that the physicist of a generation ago would 
have dismissed them as physically inconceivable. Yet 
these periods are to-day actually determined by direct 
measurement in the laboratory. 


Stability and Survival of Elements. 

Instead of regarding the hundred or less elements 
which exist to-day as manufactured, created, once for 
all time, we rather regard them as existing because they 
have survived. All other forms less stable than those 
we recognise as elements have been weeded out. Over 
sufficiently great periods of time the rarity or abundance 
of an element must be controlled by its degree of in- 
stability or stability. Probably for every stable atom 
many unstable ones could be, even are being, formed. 
But only the stable forms can accumulate in quantity 
and become known to us as ordinary chemical elements. 
We have seen that the rarest of such in all probability 
must have a period of thousands of millions of years, 
while for the more common elements, if they are chang- 
ing at all, periods of billions of years may be anticipated. 

At first glance only, the material universe gives the 
impression of a permanent and finished creation. In 
reality the now familiar remorseless operation of slow, 
continuous change moulds even " the foundation-stones " 
themselves. By this last step the doctrine of evolution 
has become universal, embracing alike the animate and 
inanimate worlds. But whereas in the former slight 
changes of environment effect the profoundest modifica- 
tions, in the latter the controlling factors still remain 
absolutely unknown. By the spectroscope a partial 
material survey of the whole universe has been ren- 
dered possible, and what we find is everywhere an essen- 
tial similarity of composition. For example, there is no 
evidence that in the sun or stars large quantities of 
elements unknown to us exist. The reason why some 
atoms are stable and others are not is a mystery we 
have not yet begun to probe. Yet this question, to us 
only of academic interest and possibly somewhat remote 
at that, will, as we shall soon come to see, be one of 
life and death to the inheritors of our civilisation. 


Connection between Range of a-RAYS and 

A very interesting development may now be men- 
tioned, which has resulted in a connection being estab- 
lished between the ranges or velocities of the various 
types of a-rays, and the periods of life of the atoms 
from which they are derived. As a general rule — not, 
it is true, entirely without exceptions, but possibly the 
exceptions may prove to be only apparent — the more 
rapidly a radioactive substance disintegrates, or the 
shorter its period of average life, the greater is the 
velocity with which the «-particle is expelled from 
the atom, and the greater therefore is the range of the 
a-particle. Thus, the most stable radio-elements, ura- 
nium and thorium, give a-rays having the lowest ranges, 
and the low range of the a-rays of ionium was for long 
the only evidence that its period must be very long. 
The greatest ranges occur in the short-lived " active 
deposit " products. The very long ranges of the a-rays 
of radium C (69-5 mm.), and of the corresponding thorium 
C (86 mm.), is generally explained by the supposition that 
the real atoms giving these rays have periods of the order 
of only a millionth of a second, and therefore that it is 
impossible to separate them from their parents, which 
thus appear to be giving rays which in reality come 
from their products. This will be referred to again. 
Latterly, this generalisation has been put into stricter 
form by the discovery that if the logarithm of the period 
is plotted against the logarithm of the range or of the 
velocity, straight lines result for each of the three known 
disintegration series. The three straight lines are 
parallel to but not identical with one another. The 
reason for this is still obscure. Some mathematical 
connection exists between the two quantities, and that 
is all that can yet be said. On the other hand, it has 
been found possible to calculate approximately some of 
the unknown periods — like that of ionium, so estimated 


at 200,000 years, for example, from the ranges of the 
a-rays by means of this relation before it was directly 
determined to be 100,000 years. 

For long it was known that uranium was exceptional 
in that it appeared to give out two a-particles per atom 
disintegrating instead of one, as in all other cases. A 
very careful investigation revealed the fact that the 
ranges of these two sets of a-particles were riot exactly 
alike. One set, those from uranium I, presumably, have 
a range of 25 mm., and the other set, those from its 
shorter-lived product, uranium II, presumably, a range 
of 29 mm. The period corresponding with 29 mm. of 
range is, in the uranium series, two million years, and 
this is the main evidence for believing that such a 
product, uranium II as' it is called, exists, and that it 
has so far not been separated from uranium because 
of the identity of the chemical properties of the two 

Pleochroic Halos. 

The account given in this chapter and in Chapter III. 
of the many extraordinary properties of the a-particle 
would be incomplete if another natural phenomenon in a 
totally distinct field were omitted. The a-, in common 
with the other rays from radioactive substances, have 
the power of darkening glass and other transparent 
materials such as mica after long exposure. Indeed, 
the colours of many natural gems have been traced to 
the effect of such rays from naturally occurring radio- 
active materials in the earth, operating over immense 
periods. Sir William Crookes artificially coloured a 
large colourless diamond an intense green by exposing 
it for some weeks to the rays from a pure radium 

Many other gems, usually found in a colourless state, 
can similarly be made to assume the most varied colours, 
the nature of which depend probably upon slight 
chemical impurities present in the gem. Mica under 
these circumstances becomes deeply stained and dark. 


Now, occurring in various natural micas, there are 
sometimes found microscopic halos of darkening of 
perfect circular outline, called pleochroic halos. These 
have been very exhaustively studied by Professor 
Joly, and the microphotographs shown in Figs. 35 and 36 
are taken from a paper by him and Mr. Fletcher in the 
Philosophical Magazine for 1910. Fig. 35 shows two of 
these halos in a specimen of mica. Sometimes the halos 
are made more visible by the use of polarised light, but 
this is not always necessary. It can be shown, by suit- 
ably sectioning the material, that the halos are true 
spheres, and often at the centre a juinute microscopic 
nucleus is visible. Professor Joly measured exactly with 
the microscope the diameter of these halos, and found 
them to correspond perfectly correctly with the " range " 
of the «-particles from radium C, which in mica is 
0-06 mm. He put forward the view that they were 
due to a-particles, from radioactive material in the 
central nucleus, darkening the mica over a sphere 
bounded by the range of the a-rays. This conclusion 
has been most brilliantly confirmed. It is possible to 
find halos in various stages of development. Young and 
incompletely developed halos often show only a central 
" pupil " of only 0-013 mm. in radius. This corresponds 
with the range of the shorter a-particles, due to uranium, 
ionium, and radium itself. In later stages a distinct 
" corona" appears of the full radius, 0-03 mm., which 
is the range of the a-particle from radium C in mica. 
And in particularly favourable cases it is possible to 
see between them an inner ring of dimensions corre- 
sponding with the intermediate range of the a-particles 
of radium A. A much enlarged micro-photograph of 
such a halo is shown in Fig. 36. 

Ueanium and Thorium Halos. 

Moreover, a careful search revealed other halos of 
slightly greater radius than 0'03 mm. — viz., 0*038 mm. — 
which corresponds with the range of the fastest a-par- 

Fig. 35. — Thorium and Radium Halos in Biotite. 
( X 150 Diameters.) 

Img. 36.— Halo in Biotite. ( x 450 Diameters.) 

Showing ring due to Radium A. 

To face p. 166 


tide emitted in the thorium series. An examination of 
them showed a course of development totally different 
from that of the uranium halos. The successive states 
in this case correspond with the a-rays of the ranges that 
are emitted in the thorium series. 

As a matter of fact the lower halo in Fig. 35 is due 
to uranium and the upper one due to thorium. The 
uranium halo is fully developed, so that the central 
" pupil," though visible in the microscope, cannot be 
seen in the reproduction. The thorium halo shows 
faintly but quite clearly the corona due to the long 
range rays of thorium C, the longest known. Still 
other halos attributed to radium emanation without 
the earlier members of the series have been observed. 

It may be concluded that the nucleus at the centre 
either contains uranium or thorium in minute quantity, 
or has the power of occluding radium emanation from 
water that has flowed through uranium minerals. But 
the actual a,mounts of radioactive materials so put into 
evidence are almost inconceivably minute and far 
beyond the power of detection even by the most sensi- 
tive electrical method. It has been estimated that the}'- 
are due to the expulsion of sometimes less than 100 
a-particles per year, continuing for several hundred 
million years. The mica integrates these infinitesimal 
effects throughout the ages so that at length they are 
able to produce consequences visible to the eye. Until 
this explanation was forthcoming, they had remained a 
complete puzzle to the petrologist. 



The Potentialities of Matter. 

This interpretation of radium is drawing to a close, but 
perhaps the more generally interesting part of it remains 
to be dealt with. We have steadily followed out the idea 
of atomic disintegration to its logical conclusions, so fai* 
as they can at present be drawn, and we have found it 
able to account for all the surprising discoveries that 
have been made in radioactivity, and capable of pre- 
dicting many, and perhaps even more unexpected, new 
ones. Let us from the point of vantage we have gained 
return to the starting-point of our inquiries and see what 
a profound change has come over it since the riddle has 
been read. Radium, a new element, giving out light 
and heat like Aladdin's lamp, apparently defying the 
law of the conservation of energy, and raising questions 
in physical science which seemed unanswerable, is no 
longer the radium we know. But although its mystery 
has vanished, its significance and importance have vastly 
gained. At first we were compelled to regard it as 
unique, dowered with potentialities and exhibiting 
peculiarities which raised it far above the ordinary run 
of common matter. The matter was the mere vehicle 
of ultra-material powers. If we now ask, why is radium 
so unique among the elements, the answer is not because 
it is dowered with any exceptional potentialities or 
because it contains any abnormal store of internal energy 
which other elements do not possess, but simply and 
solely because it is changing comparatively rapidly, 



whereas the elements before known are either changing 
not at all or so slowly that the change has been unper- 
ceived. At first sight this might seem an anti-climax. 
Yet it is not so. The truer view is that this one element 
has clothed with its own dignity the whole empire of 
common matter. The aspect which matter has pre- 
sented to us in the past is but a consummate disguise, 
concealing latent energies and hidden activities beneath 
an hitherto impenetrable mask. The ultra-material 
potentialities of radium are the common possession of 
all that world to which in our ignorance we used to refer 
as mere inanimate matter. This is the weightiest lesson 
the existence of radium has taught us, and it remains 
to consider the easy but remorseless reasoning by which 
the conclusion is arrived at. 

Why Radium is Unique. 

Two considerations will make the matter clear. In 
the first place, the radioactivity of radium at any 
moment is, strictly speaking, not a property of the mass 
of the radium at all, although it is proportional to the 
mass. The whole of the new set of properties is con- 
tributed by a very small fraction of the whole, namely, 
the part which is actually disintegrating at the moment 
of observation. The whole of the rest of the radium 
is as quiescent and inactive as any other non-radio- 
active element. In its whole chemical nature it is an 
ordinary element. The new properties are not con- 
tributed at all by the main part of the matter, but 
only by the minute fraction actually at the moment 

Let us next compare and contrast radiunt mth its 
first product, the emanation, and with its original parent, 
uranium. Uranium on the one hand, and the emanation 
on the other, represent, compared with radium, dia- 
metrically opposed extremes. Uranium is changing so 
slowly that it will last for thousands of millions of years, 
the emanation so rapidly that it lasts only a few weeks. 


while radium is intermediate with a period of average 
life of two thousand five hundred years. 

We have seen that in many ways the emanation is 
far more wonderful than radium, as the rate its energy 
is given out is relatively far greater. But this is com- 
pensated for by the far shorter time its activity lasts. 
Also, if we compared uranium with radium, we should 
say at once that radium is far more wonderful than the 
uranium, whereas in reality it is not so, as the uranium, 
changing almost infinitely more slowly, lasts almost 
infinitely longer. 

The arresting character of radium is to be ascribed 
solely to the rate at which it happens to be disintegrat- 
ing. The common element uranium, well known to 
chemists for a century before its radioactivity was sus- 
pected, is in reality even more wonderful. It is only 
very feebly radioactive, and therefore is changing 
excessively slowly, but it changes into radium, expelling 
several «-particles and so evolving large amounts of 
energy in the process. Uranium is a heavier element 
than radium, and the relative weights of the two atoms, 
which is a measure of their complexity, is as 238 is to 
226. This bottle contains about a pound of an oxide of 
uranium Avhich contains about seven-eighths of its weight 
of the element uranium. In the course of the next few 
thousand million years, so far as we can tell, it will 
change, producing over thirteen ounces of radium, and, 
in that change into radium alone, energy is given out, 
as radioactive energy, aggregating of itself an enormous 
total, while the radium produced will also change, giving 
out a further enormous aggregate quantity of energy. 

So that uranium, since it produces radium, contains 
all the energy contained in a but slightly smaller quantity 
of radium and more. It may be estimated that uranium 
evolves during complete disintegration some thirteen 
per cent, more energy than is evolved from the same 
weight of radium. But what are we to say about the 
other heavy elements — lead, bismuth, mercury, gold, 
platinum, etc. — although their atoms are not quite so 


I>eavy as uranium or radium, and although none of them, 
so far as we yet know, are disintegrating at all ? Is this 
enormous internal store of energy confined to the radio- 
active elements, that is to the few which, however 
slowly, are actually changing ? Not at all, in all 
probability. Regarded merely as chemical elements 
between radioactive elements and non-radioactive ele- 
ments, there exists so complete a parallelism that we 
cannot regard the radioactive elements as peculiar in 
possessing this internal store of energy, but only as 
peculiar in evolving it at a perceptible rate. Radium 
especially is so completely analogous in its whole 
chemical nature, and even in the character of its spec- 
trum, to the non-radioactive elements, barium, stron- 
tium, and calcium, that chemists at once placed radium 
in the same family as these latter, and the value of its 
atomic weight confirms the arrangement in the manner 
required by the Periodic Law. It appears rather that 
this internal store of energy we learned of for the first 
time in connection with radium is possessed to greater 
or lesser degree by all elements in common, and is part 
and parcel of their internal structure. 

The Total Energy evolved by Uranium. 

Let us, however, for the sake of conciseness, leave 
out of account altogether the non-radioactive elements, 
of which as yet we know nothing certainly. At least 
we cannot escape from the conclusion that the particular 
element uranium has relatively more energy stored up 
within it even than radium. Uranium is a compara- 
tively common element. The world's output per year 
is to be reckoned in tens of tons, whereas that of thorium, 
which we have still to consider, exceeds a thousand tons. 

I have already referred to the total amount of energy 
evolved by radium during the course of its complete 
change. It is about 360,000 times as much energy as is 
evolved from the same weight of coal in burning (p. 120). 
The energy evolved from uranium would be some thirteen 


per cent, greater than from the same weight of radium. 
This bottle contains about one pound of uranium oxide, 
and therefore about fourteen ounces of uranium. Its 
value is about £l. Is it not wonderful to reflect that in 
this little bottle there lies asleep and waiting to be 
evolved the energy of at least one hundred and sixty 
tons of coal ? The energy in a ton of uranium would be 
sufficient to light London for a year. The store of energy 
in uranium would be worth a thousand times as much 
as the uranium itself, if only it were under our control 
and could be harnessed to do the world's work in the 
same way as the energy in coal has been harnessed and ' 

There is, it is true, plenty of energy in the world which 
is practically valueless. The energy of the tides and of 
the waste heat from steam fall into this category as 
useless and low-grade energy. But the internal energy 
of uranium is not of this kind. The difficulty is of 
quite another character. As we have seen, we cannot 
yet artificially accelerate or influence the rate of dis- 
integration of an element, and therefore the energy in 
uranium, which requires a thousand million years to be 
evolved, is practically valueless. On the other hand, 
to increase the natural rate, and to break down uranium 
or any other element artificially, is simply transmuta- 
tion. If we could accomplish the one so we could the 
other. These two great problems, at once the oldest 
and the newest in science, are one. Transmutation of 
the elements carries with it the power to unlock the 
internal energy of matter, and the unlocking of the 
internal stores of energy in matter would, strangely 
enough, be infinitely the most important and valuable 
consequence of transmutation. 

The Importance of Transmutation. 

Let us consider in the light of present knowledge the 
problem of transmutation, and see what the attempt 
of the alchemist involved. To build up an ounce of a 


heavy element like gold from a lighter element like 
silver would require in all probability the expenditure of 
the energy of some hundreds of tons of coal, so that the 
ounce of gold would be dearly bought. On the other 
hand, if it were possible artificially to disintegrate an 
element with a heavier atom than gold and produce 
gold from it, so great an amount of energy would prob- 
ably be evolved that the gold in comparison would be of 
little account. The energy would be far more valuable 
than the gold. Although we are as ignorant as ever of 
how to set about transmutation, it cannot be denied 
that the knowledge recently gained constitutes a very 
great help towards a proper understanding of the problem 
and its ultimate accomplishment. We see clearly the 
magnitude of the task and the insufficiency of even the 
most powerful of the means at our disposal in a way not 
before appreciated, and we have now a clear perception 
of the tremendous issues at stake. Looking backwards 
at the great things science has already accomplished, 
and at the steady growth in power and fruitfulness of 
scientific method, it can scarcely be doubted that one 
day we shall come to break down and build up elements 
in the laboratory as we now break down and build up 
compounds, and the pulses of the world will then throb 
with a new source of strength as immeasurably removed 
from any we at present control as they in turn are from 
the natural resources of the human savage. 

Primitive Man and Fire. 

It is, indeed, a strange situation we are confronted 
with. The first step in the long, upward journey out 
of barbarism to civilisation which man has accom- 
plished appears to have been the art of kindling fire. 
Those savage races who remain ignorant of this art are 
regarded as on the very lowest plane. The art of kind- 
ling fire is the first step towards the control and utilisa- 
tion of those natural stores of energy on which civilisa- 
tion even now absolutely depends. Primitive man 



existed entirely on the day-to-day supply of sunlight 
for his \'ital energy, before he learned how to kindle fire 
for himself. One can imagine before this occurred that 
he became acquainted with fire and its properties from 
naturally occurring conflagrations. 

With reference to the newly recognised internal stores 
of energy in matter we stand to-day where primitive 
man first stood with regard to the energy Hberated by 
fire. We are aware of its existence solely from the 
naturally occurring manifestations in radioacti^'ity. 
At the climax of that ci^'ihsation the first step of which 
was taken in forgotten ages by primitive man, and just 
when it is becoming apparent that its ever-increasing 
needs cannot indefinitely be borne by the existing 
supphes of energy, possibilities of an entirely new 
material ci'S'ihsation are dawning with respect to which 
we find ourselves still on the lowest plane — that of on- 
lookers with no power to interfere. The energA^ which we 
require for our very existence, and which Nature supplies 
us with but grudgingly and in none too generous measure 
for our needs, is in reahty locked up in immense stores 
in the matter all around us, but the power to control 
and use it is not yet ours. ^'^Tiat sources of energ;y" we 
can and do use and control, we now regard as but the 
merest leavings of Nature's primary supphes. The 
very existence of the latter till now have remained un- 
known and unsuspected. ^Yhen we have learned how 
to transmute the elements at will the one into the other, 
then, and not till then, will the key to this hidden 
treasure-house of Nature be in our hands. At present 
we have no hint of how even to begin the quest. 

Source of Cosmical Energy. 

The question has frequently been discussed whether 
transmutation, so impossible to us, is not actually going 
on under the transcendental conditions obtaining in the 
sim and the stars. We have seen that it is actually 
going on in the world under our eyes in a few special 


cases and at a very slow rate. The possibility now 
under consideration, however, is rather that it may be 
going on universally or at least much more generally, 
and at a much more rapid rate under celestial than 
under terrestrial conditions. From the new point of 
view it may be said at once that if it were so, many of 
the difficulties previously experienced in accounting 
for the enormous and incessant dissipation of energy 
throughout the universe would disappear. 

Last century has wrought a great change in scientific 
thought as to the nature of the gigantic forces which 
have moulded the world to its present form and which 
regulated the march of events throughout the universe. 
At one time it was customary to regard the evolution 
of the globe as the result of a succession in the past 
times of mighty cataclysms and catastrophes beside 
which the eruptions of a Krakatoa or Pelee would be 
insignificant. Now, however, we regard the main 
process of moulding as due rather to ever-present, con- 
tinuous, and irresistible actions, which, though operating 
so slowly that over short periods of time their effect is 
imperceptible, yet in the epochs of the cosmical calendar 
effected changes so great and complete that the present 
features of the globe are but a passing incident of a 
continually shifting scene. Into the arena of these 
silent world-creating and destroying influences and pro- 
cesses has entered a new-comer — " Radioacti\dty " — and 
it has not required long before it has come to be recog- 
nised that in the discovery of radioactivity, or rather of 
the sub' atomic powers and processes of which radio- 
activity is merely the outward and -vdsible manifes- 
tation, we have penetrated one of Nature's innermost 

Whether or no the processes of continuous atomic 
disintegration bulk largely in the scheme of cosmical 
evolution, at least it cannot be gainsaid that these pro- 
cesses are at once powerful enough and slow enough to 
furnish a sufficient and satisfactory explanation of the 
origin of those perennial outpourings of energy by virtue 


of which the universe to-day is a going concern rather 
than a cold, Hfeless collocation of extinct worlds. Slow, 
irresistible, incessant, unalterable, so apparently feeble 
that it has been reserved to the generation in which we 
live to discover, the processes of radioactivity, when 
translated in terms of a more extended scale of space and 
time, appear already as though they well may be the 
ultimate controlling factors of physical evolution. For 
slowprocesses of this kind do the effectivework of Nature, 
and the occasional intermittent displays of Plutonic 
activity correspond merely to the creaking now and 
again of an otherwise silent mechanism that never stops. 

Radium in the Earth's Crust. 

It is one of the most pleasing features of this new 
work that geologists have been among the very first to 
recognise the applicability and importance of it in their 
science. I am not competent to deal adequately with 
or discuss the geological problems that it has raised. 
But this story would be incomplete if I did not refer, 
though it must be but briefly, to the labours of Pro- 
fessor Strutt^ who initiated the movement and to those 
of Professor Joly who has carried it on. These workers 
carried out careful analyses of the representative rocks 
in the earth's crust for the amount of radium they con- 
tained. Absolutely, the quantity of radium in common 
rocks is of course very small, although with the refined 
methods now at the disposal of investigators it is quite 
measurable. The important fact which has transpired, 
however, is that the rocks examined contain on the 
average much larger quantities of radium, and therefore 
necessarily of its original parent uranium, than might 
be expected. The amount of heat which finds its way 
in a given time from the interior of the globe to the 
surface and thence outwards into external space by 
radiation has long been accurately known. Strutt 
concluded that if there existed only a comparatively 
1 Now Lord Rayleigh. 


thin crust of rocks less than fifty miles thick of the same 
composition, as regards the content of radium, as the 
average of those he examined, the radium in them 
would supply the whole of the heat lost by the globe to 
outer space. He concluded that the surface rocks must 
form such a thin crust, and that the interior of the globe 
must be an entirely different kind of material, free from 
the presence of radium. Otherwise the world would be 
much hotter inside than is known to be the case. So 
far then as the earth is concerned, a quantity of radium 
less than in all probability actually exists would supply 
all the heat lost to outer space. So that there is no 
difficulty in accounting for the necessary source of heat 
to maintain the existing conditions of temperature on the 
earth over a period of past time as long as the uranium 
which produces the radium lasts — that is to say, for a 
period of thousands of millions of years. 

Professor Joly in his interesting work. Radio- 
activity and Geology, has considered in detail some 
of the consequences of the existence of radioactive 
materials in the earth. One of the specific instances is 
the effect of the radium in the rocks of the Simplon 
Tunnel in producing the unexpectedly high temperatures 
there encountered. From a radioactive analysis of 
these rocks he came to the conclusion that without undue 
assumptions it is possible to explain the differences in 
the temperature of the rocks encountered in boring the 
tunnel by the differences in their radium content. 

Various Possible Fates of the Earth. 

The presence in the rock of a proportion amounting 
to a few million millionths of radium above the normal 
quantity very nearly wrecked the whole enterprise. 
From the importance of radioactivity in this instance, 
of a tunnel a few miles long bored through a mountain, 
some idea may be obtained of the significance of the 
new discoveries in the general problem of the thermal 
condition of the interior of the globe. Since Strutt's 


original work, it has been established that not only 
radium, but all the other radioactive materials, includ- 
ing the whole thorium disintegratiQn series, must con- 
tribute an important quantity of heat, so that his estimate 
of a crust only fifty miles thick is in reality too great, 
and a much thinner crust would suffice. Joly has had 
the courage to push the argument to its logical conclu- 
sion, and has supposed that the radioactive materials are 
not confined to a thin surface crust, but are equally 
distributed throughout the globe in nmch the same 
proportions as they are in the crust. If this is so, there 
is no escape from the conclusion that the interior of the 
earth, so far from gradually parting with its heat and 
cooling down, must actually be getting steadily hotter. 
The heat generated within, even after the lapse of hun- 
dreds of millions of years, would scarcely appreciably 
escape from the surface, for, as Lord Kelvin deduced, 
the central core of the earth must be almost insulated 
thermally from the surface, owingto the low conductivity 
of the rocks composing the crust. He assumes through- 
out an average composition of the globe of two parts of 
radium per million million, which is considerably below 
the average he found for the rocks of the crust, and he 
calculates that in the course of a hundred million years 
this minute quantity will produce a rise of the tempera- 
ture of the central core of no less than 1,800° C. Unless, 
therefore, this heat is utilised in some unknown way, or 
the disintegration of the radio- elements is prevented by 
the high temperature and pressure, the ultimate fate 
of the globe must be very much as depicted in the 
Biblical tradition. Sooner or later the crust must 
succumb to the ever-increasing pressure within, and the 
earth must become again, what it is supposed once to 
have been, a vastly swollen globe of incandescent gas. 
As Joly remarks, there is no evidence that this has not 
already occurred more than once, nor assurance that it 
will not recur. So far as physical science yet can deduce, 
the accumulation of thermal energy within a world con- 
taining elements undergoing atomic disintegration during 


the " geological age " must alternate with a state of 
things which might be termed " the incandescent age," 
in Avhich this accumulated energy is dissipated by radia- 
tion. This periodic cycle of changes must continue until 
the elements in question have disintegrated — that is, 
over a period which radioactive measurements indicate 
is of the order of tens or hundreds of thousands of 
millions of years. During the incandescent age the loss 
of heat by radiation, which increases according to the 
fourth power of the temperature, is immensely greater 
than could be supplied even by atomic disintegration. 

Thus, if the known laws hold, it is certain that the 
present loss of heat of the sun cannot be supplied by 
the presence of radium. For this to be the case a very 
large part of the sun's 'mass must consist of uranium, 
and this we know from the spectroscope is very im- 
probable. Still, it is by no means to be concluded that 
the heat of the sun and stars is not in the first place of 
inte nal rather than, as has been the custom to regard 
it, of external origin. 

As soon as sufficient of the heat energy of a world 
has been radiated away for a solid crust to form, the 
poor thermal conductivity of this crust at once reduces 
the radiation loss to a negligible figure again, a fresh 
geological age is inaugurated, and again the heat accu- 
mulates within. This view, that the elements contain 
within themselves the energy from which Nature obtains 
her primary supplies, and that in cosmical time " geo- 
logical age " and " incandescent age " alternate as the 
night and day, however imperfect it may still be, is at 
least more in harmony with existing knowledge than the 
older conventional view that the universe was wound up 
once for all in the beginning like a clock to go for a 
certain time, for the most part quietly and uneventfully, 
pursuing its allotted path towards ultimate physical 
stagnation and death. But what a picture it conjures 
up of life and of the precariousness of its tenure^ — from 
its lowest beginnings to its highest evolution, not a 
permanent accomplishment, but a process to be inaugu- 


rated and consummated afresh, if at all, between the 
ending and beginning of each new cosmical day ! 

To escape from this conclusion it is necessary to 
suppose that atomic disintegration is cosmically not the 
inevitable uncontrollable process it has hitherto been 
proved to be under all laboratory conditions, but that 
under conditions of pressure and temperature, such as 
exist in the interior of a world, it may either be stopped 
altogether, or compensated for by unknown comple- 
mentary processes of atomic synthesis in which energy 
is taken up. 

The Most Probable View. 

The balance of probability appears to rest with the 
view that the radioactivity of the materials comprising 
the earth is confined to a crust and that the central core 
is more or less free from radioactive matter. Our 
knowledge of earthquake phenomena, and particularly 
of the three distinct routes by which an earthquake wave 
travels from one point on the surface of the earth to 
another — (1) and (2) by circular paths clockwise and 
counter-clockwise through the crust, and (3), the " P3 " 
route, by a straight line joining the two points — has 
strongly supported the view that the core of the earth 
is of a totally different nature from the crust. On the 
P3 route, once the wave gets below the crust, it travels 
much faster than it does through the surface. This, 
especially, confirms the picture of the earth as a metallic 
sphere of nickel-steel within, surrounded with a thin 
surface layer of solidified slag, which its high specific 
gravity and the composition of meteorites first sug- 
gested. On this view, it is to be expected that the 
radioactive materials will be confined to the crust and 
be absent from the metallic core, and, therefore, that 
the crust may have reached a steady temperature, at 
which the loss of heat by radiation is exactly balanced 
by the heat evolved by its radioactive constituents. If 
this is so, the present state would continue without 
much change for hundreds of millions of years. 


Be that as it may, our outlook on the physical uni- 
verse has been permanently altered. We are no longer 
the inhabitants of a universe slowly dying from the 
physical exhaustion of its energy, but of a universe 
which has in the internal energy of its material compo- 
nents the means to rejuvenate itself perennially over 
immense periods of time, intermittently and catastro- 
phically, which is the first possibility that presents itself, 
or continuously and in orderly fashion, if there exist 
compensating phenomena still outside the ken of science. 

Radioactivity and Mythology. 

The world probably being of much greater antiquity 
than physical science has thought to be possible, it is 
interesting and harmless to speculate whether man has 
shared with the world its more remote history. 

In this connection it is curious how strangely some 
of the old niyths and legends about matter and man 
appear in the light of the recent knowledge. Consider, 
for example, the ancient mystic symbol of matter, 
known as Ouroboros — " the tail devourer " — which was 
a serpent, coiled into a circle with the head devouring 
the tail, and bearing the central motto, " The whole is 
one." This symbolises evolution; moreover, it is evolu- 
tion of matter — the very latest aspect of evolution — the 
existence of which was strenuously denied by Clerk 
Maxwell and others of only last century. The idea which 
arises in one's mind as the most attractive and consistent 
explanation of the universe in the light of present know- 
ledge is, perhaps, that matter is breaking down and its 
energy being evolved and degraded in one part of a cycle 
of evolution, and in another part, still unknown to us, 
the matter is being again built up with the utilisation^ 
of the waste energy. If one wished to symbolise such 
an idea, in what better way could it be done than by the 
ancient tail-devouring serpent ? 

Some of the beliefs and legends which have come 
down to us from antiquity are so universal and deep- 


rooted that we are accustomed to consider them almost 
as old as the race itself. One is tempted to inquire how 
far the unsuspected aptness of some of these beliefs and 
sayings to the point of view so recently disclosed is the 
result of mere chance or coincidence, and how far it may 
be evidence of a wholly unknown and unsuspected 
ancient civilisation of which all other relic has dis- 
appeared. It is curious to reflect, for example, upon 
the remarkable legend of the philosopher's stone, one of 
the oldest and most universal beliefs, the origin of which, 
however far back we penetrate into the records of the 
past, we do not probably trace to its real source. The 
philosopher's stone was accredited the power not only 
of transmuting the metals, but of acting as the elixir 
f>f W^' Now, whatever the origin of this apparently 
meaningless jumble of ideas may have been, it is really 
a perfect and but very slightly allegorical expression of 
the actual present views we hold to-day. It does not 
require much effort of the imagination to see in energy 
the life of the physical universe, and the key to the 
primarjT^ fountains of the physical life of the universe 
to-day is known to be transmutation. Is, then, this old 
association of the power of transmutation with the 
elixir of life merely a coincidence ? I prefer to believe 
it may be an echo from one of many previous epochs in 
the unrecorded history of the world, of an age of men 
which have trod before the road we are treading to-day, 
in a past possibly so remote that even the very atoms 
of its civilisation literally have had time to disintegrate. 
Let us give the imagination a moment's further free 
scope in this direction, however, before closing. What 
if this point of view that has now suggested itself is 
true, and we may trust ourselves to the slender founda- 
tion afforded by the traditions and superstitions which 
have been handed down to us from a prehistoric time ? 
Can we not read into them some justification for the 
belief that some former forgotten race of men attained 
not only to the knowledge we have so recently won, but 
also to the power that is not yet ours ? Science has 


reconstructed the story of the past as one of a con- 
tinuous Ascent of Man to the present-day level of his 
powers. In face of the circumstantial evidence existing 
of this steady upward progress of the race, the tradi- 
tional view of the Fall of Man from a higher former state 
has come to be more and more difficult to understand. 
From our new standpoint the two points of view are by 
no means so irreconcilable as they appeared. A race 
which could transmute matter would have little need 
to earn its bread by the sweat of its brow. If we can 
judge from what our engineers accomplish with their 
comparatively restricted supplies of energy, such a race 
could transform a desert continent, thaw the frozen poles, 
and make the whole world one smiling Garden of Eden. 
Possibly they could explore the outer realms of space, 
emigrating to more favourable worlds as the superfluous 
to-day emigrate to more favourable continents. The 
legend of the Fall of Man, possibly, may be all that has 
survived of such a time before, for some unknown reason, 
the whole world was plunged back again under the 
undisputed sway of Nature, to begin once more its 
upward toilsome journey through the ages. 

The New Prospect. 

The vistas of new thought which have opened out in 
all directions in the physical sciences, to which man is 
merely incidental and external, must in turn react 
powerfully upon those departments of thought in which 
man is central and supreme. We find ourselves in con- 
sequence of the progress of physical science at the pin- 
nacle of one ascent of civilisation, taking the first step 
upwards out on to the lowest plane of the next. Above 
us still rises indefinitely the ascent to physical power — 
far beyond the dreams of mortals in any previous system 
of philosophy. These possibilities of a newer order of 
things, of a more exalted material destiny than any 
which have been foretold, are not the promise of another 
world. They exist in this, to be fought and struggled 


for in the old familiar way, to be wrung from the grip of 
Nature, as all our achievements and civilisation have, 
in the past, been wrung by the labour of the collective 
brain of mankind guiding, directing, and multiplying 
the individual's puny power. This is the message of 
hope and inspiration to the race which radium has con- 
tributed to the great problems of existence. No attempt 
at presentation of this new subject could be considered 
complete which did not, however imperfectly, suggest 
something of this side. It is fitting to attempt to see 
how far purely physical considerations will take us in 
delimiting the major controlling influences which regu- 
late our existence. 

Surveying the long chequered, but on the whole con- 
tinuous, ascent of man from primeval conditions to the 
summit of his present-day powers, what has it all been at 
bottom but a fight with Nature for energy — for that 
ordinary physical energy of which we have said so much ? 
Physical science sums up accurately in that one generali- 
sation the most fundamental aspect of life in the sense 
already defined. 

Of course life depends also on a continual supply of 
matter as well as on a continual supply of energy, but 
the struggle for physical energy is probably the more 
fundamental and general aspect of existence in all its 
forms. The same matter, the same chemical elements, 
serve the purposes of life over and over again, but the 
supply of fresh energy must be continuous. By the law 
of the availability of energy, which, whether universal 
or not, applies universally within our own experience, 
the transformations of energy which occur in Nature 
are invariably in the one direction, the more available 
forms passing into the waste and useless unavailable 
kind, and this process, so far as we yet know, is never 
reversed. The same energy is available but once. The 
struggle for existence is at the bottom a continuous 
struggle for fresh physical energy. 

This is as far as the knowledge available last century 
went. What is now the case ? The aboriginal savage, 


ignorant of agriculture and of the means of kindling fire, 
perished from cold and hunger unless he subsisted as a 
beast of prey and succeeded in plundering and devouring 
other animals. Although the potentialities of warmth 
and food existed all round him, and must have been 
known to him from natural processes, he knew not yet 
how to use them for his own purposes. It is much the 
same to-day. With all our civilisation, we still subsist, 
struggling among ourselves for a sufficiency of the 
limited supply of physical energy available, while all 
around are vast potentialities of the means of susten- 
ance, we know of from naturally occurring processes, but 
do not yet know how to use or control. Radium has 
taught us that there is no limit to the amount of energy 
in the world available to support life, save only the limit 
imposed by the boundaries of knowledge. 

It cannot be denied that, so far as the future is con- 
cerned, an entirely new prospect has been opened up. 
By these achievements of experimental science Man's 
inheritance has increased, his aspirations have been up- 
lifted, and his destiny has been ennobled to an extent 
beyond our present power to foretell. The real wealth 
of the world is its energy, and by these discoveries it, 
for the first time, transpires that the hard struggle for 
existence on the bare leavings of natural energy in which 
the race has evolved is no longer the only possible or 
enduring lot of Man. It is a legitimate aspiration to 
believe that one day he will attain the power to regulate 
for his own purposes the primary fountains of energy 
which Nature now so jealously conserves for the future. 
The fulfilment of this aspiration is, no doubt, far off, 
but the possibility alters somewhat the relation of Man 
to his environment, and adds a dignity of its own to 
the actualities of existence. 




The Thorium Disintegeation Series. 

Those who have mastered the intricacies of the uranium 
disintegration series may wish to know something of the 
important developments which have taken place since 
these lectures were first given in 1908, and of the other 
two great disintegration series known to science, the 
thorium and the actinium series. Space precludes a 
description as detailed and non-technical as that before 
aimed at, and in some of the more difficult sections it 
will be necessary to assume a considerable knowledge 
on the part of the reader of physical and chemical 
science. But the attempt seems worth making for the 
sake of completeness. 

The thorium disintegration series is becoming in- 
creasingly important, and its consideration does not 
involve any new principles. Thorium is an element 
which was at one time rare and little known even to 
chemists, but has come into prominence during the last 
twenty years, because of its use as a constituent of the 
incandescent gas-mantle, which is composed of about 
99 per cent, of thorium oxide, and 1 per cent, of cerium 
oxide. Fairly abundant sources of thorium have been 
discovered in the sands of certain coasts in Brazil, 
North and South Carolina, etc., where a natural con- 
centration has taken place by the action of the sea- 
waves of the particles of the heavy mineral monazite, 



which occurs as a minute constituent in many rocks, 
and in the sands derived from them by the action of 
weathering agencies. The monazite is concentrated 
from the sand usually by magnetic methods, until it 
contains 4 per cent, of thorium oxide. This constitutes 
the monazite sand of commerce, and from it every year 
hundreds of tons of pure thorium salts are now manu- 
factured for the gas-mantle industry. More recently the 
find has been made of a very rich monazite in Central 
India, containing nearly 10 per cent, of thorium. 

Mesothorium and Radiothorium. 

As already described, the usual a-radioactivity of 
commercial thorium compounds is of about the same 
strength as that of pure uranium compounds, but the 
/3- and 7-, or penetrating activity, is several times less 
intense. We have seen (p. 153) that in the uranium 
minerals, although several intermediate products of the 
disintegration of uranium are present, with periods of 
life sufficiently long, and radioactivity sufficiently im- 
portant, to repay extraction, it is practicable to extract 
only one of these — namely, radium. In the thorium 
minerals there are two such products, named meso- 
thorium and radiothorium, and though their periods of 
average life, about eight years and three years respec- 
tively, are very much less than that of radium, they are 
sufficiently long to make their extraction and utilisation 
practicable. Whereas the sources of radium are costly 
and comparatively limited in amount, the by-products 
of the thorium industry, after the extraction of the 
technically valuable thorium, are the source from which 
mesothorium and radiothorium are extracted. Much 
greater quantities of these by-products have to be 
handled, it is true, than in the extraction even of radium 
from pitchblende to produce similar results. The new 
substances must, on this account, always be costly to 
produce. But in the by-products of a single year's 
manufacture of thorium the new products capable of 


being extracted possess as much radioactivity as at 
least an ounce of pure radium. They thus offer an 
abundant source of radioactive material, which at present 
is mostly wasted, and the product, while it lasts, is in 
every respect the equal of radium in its properties. The 
only disadvantage it possesses is its relatively much 
shorter period of life. 

The discoveries in the thorium series of these two 
technically valuable members were made by Otto Hahn, 
who has worked both with Sir William Ramsay and Sir 
Ernest Rutherford, comparatively recently, after the 
rest of the members had become quite well known. The 
historical development of the subject from the first dis- 
covery of the radioactivity of thorium compounds up 
to the present time is a most interesting chapter to the 
student, but would unduly complicate the subject if 
considered here. It is better to proceed in order through 
the thorium disintegration series as it is at present 
known, apart from historical considerations as to the 
order in which they were discovered, though, as in 
the case of the uranium series, the first members were 
the last to be separately recognised. 

Radioactivity of Thorium. 

Unlike pure uranium salts, which, a few months after 
preparation, have a definite constant radioactivity, 
consisting of all three types of rays, the a-activity being 
due to uranium, and the 13- and 7-activity to the short- 
lived uranium X in equilibrium with it, thorium salts, 
though chemically pure, vary continuously in their whole 
radioactivity for twenty or thirty years after manu- 
facture. Even after these periods, slight changes must 
still be going on, and probably fifty years would have 
to elapse before they became quite inappreciable. But 
in spite of the great apparent differences between the two 
elements, there is a very close analogy in their disin- 
tegration series, every one of the eleven known members 
of the thorium series having an analogue in the twelve 


members of the uranium series as far as radium D, at 
which point the thorium disintegration appears to come 
to an end. One a.-ray giving product in the uranium 
series is not represented in the thorium series. The 
analogous members in the two series usually give out 
similar kinds of rays, and although their periods are 
often widely different, there is a rough correspondence in 
the two series between the relative periods of the suc- 
cessive members, the periods in the thorium series being, 
however, usually much less than in the uranium series. 
Thus uranium I, with its period of 8,000,000 years, gives 
a-rays, and is followed by uranium X^, giving (/3)-rays, 
of period 35-5 days, and by uranium X2, or brevium, 
of very short period, which gives powerful and penetrat- 
ing /S-rays. Uranium II,' which follows, is chemically 
identical with uranium I, and, like it, is of long period 
and gives a-rays. This produces ionium, which gives 
«-rays, and has the period of 100,000 years. Ionium, in 
turn, produces radium, which gives a-rays, and has a 
period of 2,500 years. Thorium itself is provisionally 
estimated to have a period about three times longer than 
uranium I, and gives only a-rays. It produces by its 
disintegration " mesothorium I," which does not give 
any important rays, and has a period of 7-9 years. It 
is identical in chemical character with radium, and 
corresponds with uranium X^, except that no /S-rays at 
all are expelled. Its product is called " mesothorium 
II," which corresponds very closely with uranium Xg, 
giving out powerful yS- and 7-rays, and having a period 
of only 8-9 hours. It produces in turn " radiothorium, " 
which corresponds perfectly with ionium, giving a-rays, 
and having a period of 2-9 years. These last two sub- 
stances are chemically identical with one another, and 
also with thorium itself, and cannot be separated by any 
known method when mixed together. This fact is of 
considerable importance, as thorium when separated 
from a mineral, always contains at first all the radio- 
thorium in the mineral and also all the ionium, if ura- 
nium was also present, as is almost invariably the case. 



The product of radiothorium is thorium X, which corre- 
sponds with radium, giving a-rays, but having a period 
of only 5-6 days. Thorium X is chemically identical 
with radium, and also with mesothorium I. This 
chemical identity of radium and mesothorium I is the 
dominating fact in the separation of these new sub- 
stances, as will later be more clear. After thorium X, 
the thorium emanation results, corresponding perfectly 
in its whole nature as a member of the argon family of 
gases, with the radium emanation, and giving a-rays, 
but having the much shorter period of only 76 seconds. 
Its product is the thorium active deposit, of which the 

Meso- Meao- Radio- Thorium X. 

thorium I. thorium II. thorium. 

25,000,000,000 7-9 years. 8-9 hours. 2-91 years. 5-35 days. 
(?) years. 

Emanation. Thorium A. Thorium B. Thorium C, Thorium D. Thorium E. 
76 seconds. 0-2 second. 16'3 hours. 79 minutes. 4-5 minutes. (Lead.) 

Fig. 37. 

first three members, called thorium A, B, C, are almost 
precisely analogous to the corresponding radium mem- 
bers, except in period. The period of thorium A is only 
one-fifth of a second. Those of thorium B and C are 
15-3 hours and 79 minutes respectively. These last are 
the only two, except thorium itself, possessing periods 
longer than the corresponding members of the uranium 
series. Lastly, there exists, as the product of thorium 
C, thorium D, the last active member known, which gives 
^- and 7-rays, and has the short period of 4-5 minutes. 
It has little analogy to radium D. The ultimate pro- 
duct of thorium was till recently not even guessed. All 
that could be said is that its atomic weight, calculated 


from that of thorium and the number of a-particles ex- 
pelled, is 208, and this is the atomic weight of bismuth ! 
It cannot be bismuth, because in some ancient thorium 
minerals hardly a trace of bismuth can be found. The 
discovery of its nature came as a surprise, for in spite of 
the difference of atomic weight, it proves to be the same 
element as ends the uranium series — namely, lead. This 
has raised very deep issues. The complete thorium 
disintegration series is shown on p. 190 (Fig. 37), so far as 
we have yet considered it. But thorium C, there shown 
single, is like radium C complex (see p. 201). 

The extraordinary analogies between this series and 
the uranium series, on the one hand, and the actinium 
series, on the other, will later receive a very satisfying 


It is clear that mesothorium I, with the period of 
average life of nearly eight years, being both the first 
and the longest lived of the successive products, is the 
centre of interest. The radioactivity of the element 
thorium itself, consisting only of low-range a-rays, of 
relatively feeble intensity because of the enormous period 
of the element, is technically and scientifically even of 
less interest than that of uranium. Mesothorium, how- 
ever, corresponds to radium in that it can be concen- 
trated, and the greater part of the radioactivity of many 
tons of minerals can be separated in a preparation weigh- 
ing less than a few milligrams. Just as when radium is 
first prepared it gives only the relatively unimportant 
a-activity proper to itself, but in course of time develops 
enormously in all its activity due to the growth and 
accumulation of its products, so it is with mesothorium. 
Freshly prepared and free from its products, it has 
practically no activity. In the course of a few hours 
the strong penetrating activity of its short-lived product, 
mesothorium II, develops, and in two or three days this 
reaches a maximum or equilibrium value. This part of 
the activity then remains, so long as the preparation is 


not chemically treated, apparently constant, but actually 
decaying very slowly. This decay is to half the initial 
value after 5-5 years, to a quarter after 11 years, and so on. 
But the product of this change is radiothorium, which 
gives cc-rays; and, just as in the case of radium, this is 
followed by a small host of short-lived products, some of 
which give a- and others /3- and 7-rays. What actually 
happens, therefore, is that in addition to the initial 
rapid growth of /S- and 7-rays already discussed, a slow 
steady increase of the a-, /3-, and 7-activity of a meso- 
thorium preparation takes place for many years after its 
preparation, due to the growth and accumulation of 
radiothorium and its products. It is calculated that 
this increase will go on for about four and a half years, 
and then the activity of the preparation will reach a 
maximum, the penetrating activity (yS- and 7-rays) 
being then nearly twice that at two days after prepara- 
tion. From then onwards the regular slow decay of all 
the radioactivity will set in, and continue with the half- 
period of five and a half years, as already considered. 
Twenty years after preparation the activity will be 
some 12 per cent., whilst after a century it would be less 
than 1,000th per cent, of the maximum activity. 

In practice, however, the change is even more com- 
plicated than this on account of the invariable presence 
of radium in the mesothorium preparations. Radium 
and mesothorium form, as already remarked, an example 
of which now so many exist in radioactivity, of two 
different elements, having entirely different radioactive, 
but entirely identical, chemical character. For a long 
time the nature of the chemical processes used to extract 
mesothorium from the by-products of monazite was kept 
secret. It was thought that they were peculiarly diffi- 
cult and forbidding. I was therefore surprised and 
interested to find — and the same discovery was made 
at about the same time by Professor Marckwald in Berlin 
— that mesothorium and radium behaved in chemical 
processes identically. In consequence the extraction 
of mesothorium from monazite residues is entirely 


similar in principle to that of radium from pitchblende 
residues. Since monazite always contains a minute 
amount of uranium, and therefore the corresponding 
quantity of radium, the mesothorium separated always 
contains the radium also. No successful separation has 
as yet been achieved, and it is most improbable that it 
ever will be. After a lengthy fractional crystallisation 
of the mixture I found the relative proportions of the 
two elements entirely unaltered. Technical meso- 
thorium owes about 12 per cent, of what has been 
termed its maximum activity (that after four and a half 
years) to radium. This activity will remain when all 
that due to mesothorium has completely decayed away. 
In practice, therefore, the decay of the preparations will 
be appreciably slower than if radium were absent. 

These discoveries have thus resulted in the provision 
of an effective substitute for radium, which for such 
purposes as medical application, or for general researches 
in the properties of the new radiations, are, while the 
activity lasts, its equal in every respect. Indeed, it is 
possible to obtain mesothorium preparations many 
times more concentrated in their activity than pure 
radium salts. There is no dearth of the raw material, 
which hitherto has been a wasted product. 

But, of course, from the strictly scientific point of 
view, the radioactivity of mesothorium is as distinct 
from that of radium as copper is from zinc, or as one 
flower is from another. It will be of interest to con- 
centrate upon some of the chief resemblances and differ- 
ences in the two disintegration series. 

The Thorium Emanation. 

The thorium emanation was the first of the three 
emanations to be discovered, and had been fairly com- 
pletely investigated by Rutherford before the others 
were known. It is given off in greater or less degree by 
all thorium compounds. If the radioactivity of the 
compound is measured by placing it in a closed electro- 


scope, the activity is found to increase for about ten 
minutes, owing to the accumulation of the emanation, 
and then remains constant if the instrument is not dis- 
turbed. But if a current of air is blown through the 
instrument, it sweeps out the emanation, and the acti- 
vity is correspondingly reduced. On stopping the blast 
of air, it rises again precisely as at first. Uranium com- 
pounds show no trace of this behaviour, as they do not 
generate an emanation. The products of the disintegra- 
tion of the thorium emanation are known as the thorium 
active deposit, and they manifest themselves in much the 
same way as the radium active deposit, being attracted 
to the negatively charged surface in an electric field. 
They last much longer, however, the period of half- 
decay being about eleven hours instead of about half an 
hour, and, in consequence, they take longer to accu- 
mulate. In a vessel containing a thorium or, better, a 
radiothorium preparation, which acts as a constant 
source of the evanescent thorium emanation, the active 
deposit on the walls of the vessel (or on the negative 
electrode, if an electric field is used), goes on increasing 
in amount for about two days, whereas in the radium 
emanation the active deposit reaches the maximum 
value in about three hours. 


Radiothorium is the most powerful and convenient 
source of the emanation and active deposit of thorium. 
As already explained, radiothorium is not separable 
from thorium by any chemical process. Freshly pre- 
pared thorium compounds contain practically all the 
radiothorium of the original mineral, but its parent meso- 
thorium being absent, this radiothorium in the course 
of a few years decays. Before it decays completely, 
however, mesothorium has been regenerated by the 
thorium, and in time begins to produce fresh radio- 
thorium. The consequence is that commercial thorium 
compounds always contain more or less radiothorium, 


and always, therefore, furnish more or less of the emana- 
tion and active deposit. But the amount is insignificant 
compared with what can now be obtained from a com- 
mercial radiothorium preparation. Mesothorium, after 
it is separated from the mineral and left to itself, pro- 
duces, as we have seen, radiothorium. After a year or 
more of accumulation these two substances may with 
advantage be separated. A trace of a thorium salt is 
added to the solution, and then precipitated by adding 
ammonia as thorium hydroxide, which carries with it the 
whole of tha radiothorium, leaving the mesothorium in 
solution. This radiothorium preparation in turn gener- 
ates thorium X, and after a few weeks becomes a power- 
ful source of the thorium emanation during the few years 
it lasts. 

Apart from the intrinsic interest attaching to this 
method of " growing " radio-elements otherwise not 
separable from the raw material a point of great 
philosophical interest is involved. Were it not for the 
existence of mesothorium intermediate between and 
chemically distinct from thorium and radiothorium, the 
separate existence of the latter might not have been 
suspected, and they certainly could never be obtained 
as individuals. In the case of uranium I and uranium 
II, the evidence for the existence of two elements remains 
indirect, and they have never yet been separated. The 
intervening member, uranium X, is too short-lived and 
the product uranium II too long-lived for the quantity 
of the latter produced from the former to be detectable 
even by radioactive methods (vide pp. 129 and 150). One 
can hardly help wondering how many of the well-known 
common so-called elements may not be mixtures of more 
than one element with chemically identical properties. 

Experiments with the Thorium Emanation. 

Radiothorium may be used to show, in a striking way, 
by means of phosphorescent screens, many of the older 
classical experiments on the growth and decay of radio- 


active substances on which the existing theory of atomic 
disintegration has been built up. For example, if a 
radiothorium preparation or old mesothorium prepara- 
tion containing radiothorium, is kept in a tube through 
which a puff of air can be sent from a rubber blower, 
and the accumulated emanation is thus blown out into 
a flask internally coated with zinc sulphide, as shown in 
Figs. 8 and 9, it will cause it to phosphoresce briJliantly 
in the dark. The decay of the emanation in the flask 
can then be watched from minute to minute with the 
eyes, and its concomitant reproduction in the radio- 
thorium tube can easily be demonstrated. For example, 
the radiothorium tube may first be thoroughly blown 
out, and then the effect observed of blowing through it 
into a zinc sulphide flask immediately, before any emana 
tion has had time to accumulate, and then after waiting 
successive periods of, say, ten, twenty, thirty seconds, 
one, two, ten, or more minutes. For the shorter intervals 
the amount of emanation produced is very nearly pro- 
portional to the time, but for the longer ones the decay 
of that produced first during the period of accumulation 
begins to tell, and the increase with time gets less and 
less. So that after five or ten minutes no increase 
results, however long a time is allowed to elapse. The 
emanation is then in " equilibrium," as much decaying 
per second as is produced per second. 

In this way many of the simple laws of the decay and 
reproduction of the emanation, on which the whole super- 
structure of radioactive theory was at first largely based, 
may now be shown to a large audience. But all the 
original work was done with delicate electrical instru- 
ments long before anyone had ever observed a single 
visible effect, or had any other than indirect elec- 
trical evidence of the existence of the evanescent 


Thorium A. 

The most recent member to be added to the thorium 
disintegration series is thorium A, the direct product 
of the emanation, which, on account of its short period 
of average Hfe — about one-fifth of a second only — had 
hitherto not been separately distinguished from the 
emanation. It was put in evidence by Rutherford and 
his colleagues in the following ingenious manner: An 
endless wire passed along the axis of a cylinder, con- 
taining a radiothorium preparation, through holes in 
ebonite stoppers closing the ends of the cylinder, and 
over suitable pulleys outside of the cylinder driven by 
an electric motor. In this way the wire was kept passing 
through a cylinder filled with thorium emanation. The 
wire was connected to the negative and the cylinder to 
the positive pole of a battery, so as to concentrate the 
active deposit on the wire. It was found that the wire 
immediately after leaving the cylinder was intensely 
active, giving out powerful a-rays, and capable of light- 
ing up a zinc sulphide screen brought near to the wire. 
This activity on the wire lasts only a small fraction of a 
second, so that after the wire has moved away a little 
from the cylinder its activity has practically disappeared. 
Thus, although the wire is being driven at a high speed 
all the time, it is only the part immediately issuing from 
the cylinder which is active, and which causes the sul- 
phide screen to glow. Thorium A is a non-volatile 
product of the gaseous emanation, and is attracted to 
the negative electrode. But almost as soon as it is 
deposited it breaks up, giving «-rays. On the principle 
of a short life and a merry one, the effect of this product 
is far more marked, for short periods of exposure, than 
that of the longer-lived products it in turn produces. 
Though it would be easy to show that the wire, after the 
large activity due to thorium A is over, still possesses 
activity due to the products formed, this activity is, 
for short periods of exposure, far too small to light up a 


phosphorescent screen. In this way the existence of 
this almost hopelessly unstable element has been demon- 
strated. In connection with the thorium active deposit 
and the complex character of thorium C something has 
still to be said, but it may be deferred. 

The Actinium Disintegration Series. 

A few words may be said for the sake of completeness 
on the third and least important disintegration series, 
but one which is, however, just as interesting to the 
student as the others. In addition to the polonium and 
radium separated from pitchblende by M. and Mme. 
Curie, a colleague, M. Debierne, was successful in isolat- 
ing a third new radio-element, to which he gave the 
name Actinium. So far as is known, actinium is at 
least a fairly long-lived radio-element, for although it 
was discovered very shortly after radium, the original 
preparations have retained much, at least, of their 
activity. Recently it has been established that a slow 
decay, however, does occur which indicates a period of 
average life for this substance of only about thirty years. 

Actinium is separated with the " rare earths " in 
uranium minerals, and chemically it resembles most 
closely the rare-earth element, lanthanum, although it 
is not completely identical with it in chemical pro- 
perties. In radioactive properties its disintegration 
series is very closely analogous to that of thorium, and 
consists of eight members, in addition to itself, the first 
of which, known as radioactinium, corresponds with 
radiothorium. The next is actinium X, corresponding 
perfectly with thorium X, and after tPiat the actinium 
emanation, actinium A, B, C, and D, follow in regular 
order, almost exactly as in the thorium series. The 
analogous products in the two series in each case give 
out the same kinds of rays, and are, so far as is known, 
chemically identical in character. But, almost without 
exception, the periods in the actinium series are uni- 
formly shorter than in the thorium series, the longest, 


that of radioactinium, being only twenty-eight days, 
and the shortest, that of actinium A, being only ^oth 
of a second. The full series is shown in Fig. 38. 

O^ D^ O^ O^ 


Actinium. Badio- Actinium X. Emanation. Actinium A. 

SOyeai-s. 28-1 days. 15 days. 5-6 seconds. 0'003 second 

Actinium Actinium Actinium Actinium 

B. C. D. E. 

52-1 minutes. 3'1 minutes. 6*83 minutes, (unknown). 

Fig. 38. 

The Origin of Actinium. 

Whereas it is customary to regard the uranium and 
thorium series each as starting from a primary parent of 
so long Ufe, that, old as the world is, some still survives 
unchanged, the problems connected with the real nature 
and origin of actinium are still not entirely cleared up. 
Its short period of life, recently established, proves that 
it cannot itself be a primary radio-element like the 
other two, and, in fact, its parent is now known. But 
it is not impossible that it may form part of a third 
independent primary series, though this has not been 
the view that has so far gained most support. So far as 
knowledge has been gained, actinium appears to be found 
only in the uranium minerals and in all of these which 
have been examined for it. It is natural to conclude 
from this that it is a product of uranium. But here a 
difficulty arises. In the disintegration of actinium at 
least five a-particles are given out per atom disintegrat- 
ing, representing a loss in atomic weight of 20 units. 
There is certainly no room for the actinium series between 
uranium and polonium, and there is no evidence that it 
comes after polonium. 


Multiple Atomic Disintegration. 

The important piece of evidence, however, which 
shows conclusively that actinium cannot be in the main 
uranium-polonium series, and which at the same time 
serves to distinguish this series from the others, and to 
make it practically the most difficult to investigate, is 
the extraordinarily small relative quantity of actinium 
in uranium minerals. Although the actinium series 
gives out at least five a-particles per atom as compared 
with eight given out by the uranium-radium-polonium 
series, the a-radiation contributed by the whole actinium 
series in uranium minerals is only about one-fifteenth or 
one-sixteenth of that contributed by the uranium series. 
Whereas, if actinium were in the main line of descent 
from uranium, the a-activity of its series should be of 
the order of five-eights of that of the uranium series, in 
accordance with the principle discussed on p. 153. Two 
possibilities may be advanced. Either actinium is an 
entirely separate and independent primary radio- 
element, and, if so, its occurrence always in uranium 
minerals, and only in those minerals, is difficult to 
understand; or actinium may be derived from uranium 
as a branch, or offshoot, not in the main line of descent. 
One may suppose that at some stage of the disintegra- 
tion of the uranium atom a choice of two modes of dis- 
integration presents itself. The large majority of the 
atoms choose one way — the way leading to polonium — 
whilst a small minority choose a second way, the way 
leading through the -actinium series. If this is true, it 
can be calculated that out of every twelve uranium 
atoms, eleven go through the main line of descent to- 
wards polonium, and one goes through the actinium line. 
This mode of explaining actinium is now supported by 
much new evidence and by the discovery very recently 
of actual cases of such a multiple disintegration at 
the ends of the thorium and radium series, among the 
active deposit products. 


Branch Series of Thorium and Radium. 

This may now be briefly dealt with. We have already 
considered the evidence (p. 164) for supposing that, on 
account of the very high speed at which the a-particles 
are expelled from radium C and thorium C, the changes 
of these substances are complex, and that the a-rays in 
each case probably result from subsequent products, 
named radium C and thorium C, of excessively short 
life-period, which is estimated to be one millionth of a 
second in the case of the former and one hundred 
thousand millionth of a second in the case of the latter. 
In addition, the changes are complicated by branchings 
of the kind just considered, but especially instructive. 
Taking the case of thorium C first, it is known that it 
breaks up in two ways. In the first mode an a-ray is 
expelled and the product formed then expels a yS-ray. 
In the second mode the order is reversed, the /3-ray 
being expelled first and the a-ray second. This may 
be represented (see Fig. 39). 

Range 4-55 cm. 

/ ^^ 

O Range 8-i6cm. 

©— o 

121-5 min. io~"'secs. 

Branching of the Thorium Series. 
Fig. 39. 

About 35 per cent, of the atoms disintegrating follow 
the first mode producing thorium D, and give out a-rays 
of range 4*55 cm. ; whereas 65 per cent, give out /3-rays 
in the second mode producing the hypothetical and 
hopelessly evanescent thorium C, which gives out 


a-rays of range 8-16 cm. The two end products are 
of the same atomic weight, 208, and whether or not 
they are really identical cannot yet be said. The two 
separate periods of average life for thorium C shown in 
the figure, 225-7 and 121-5 minutes, are those calculated 
for the two kinds of change separately, assuming that 
the other did not occur. 

In the case of radium C, the same applies with the 
exception that only 0-03 per cent, of the atoms follow 
the " «- then jS-mode," the overwhelming preponder- 
ance, 99-97 per cent., following the " /3- then a-mode " 
(see Fig. 40). 

10"^ sees. z4 years 


Q y ") Calculated range 

28.1mm. Kja #^ '^ 

V / 

6.5 days 1-9 min. 

Branching of the Radium Series 
Fig. 40. 

The product produced in the first mode, called 
radium Cg is in such infinitesimal quantity, that little 
is known about it beyond the value of its period and the 
fact that it gives /3-rays. 

The Actinium Branch Series. 

Reverting now to actinium, the practical consequence 
of its being formed only in the minor mode of a dual 
disintegration, claiming only some 8 per cent, of the 
uranium atoms disintegrating, is that the substance is 
very much rarer and more difficult to obtain than the 
members of either of the other two series. If it were 
more common, it would lend itself to many demonstra- 


tions and experiments similar to those detailed under 
radiothorium, but of an even more striking character. 
Actinium is relatively poor in penetrating rays, and even 
the most active preparations it is possible to procure 
are disappointing in this respect when compared with 

The Actinium Emanation. 

The chief glory of actinium, however, is its emanation, 
a gaseous disintegration product, precisely analogous 
in every respect to those of radium and of thorium, but 
having a period of average life of only 5-6 seconds. The 
usual principle of a short life and a merry one applies. 
The dominating characteristic of the radioactivity of 
actinium preparations is the emanation that is given off. 
In the dark room, if a preparation is held over a zinc 
sulphide screen, the emanation diffusing away lights up 
the screen in patches, which are wafted from one part 
of the screen to another by draughts or any gentle puffs 
of air. The rapid decay of the emanation and corre- 
sponding rapid regeneration from the actinium prepara- 
tion makes it quite possible to experiment thus with 
the emanation in the open-air of the room. Whereas if 
the radium emanation were dealt with in this way, once 
it had been dissipated throughout the air of a room, 
some weeks would have to elapse before a fresh supply 
was available. Giesel, who rediscovered the substance 
subsequently to Debierne, actually named it " Ema- 
nium " before it was found to be identical with actinium. 

Actinium A. 

The only other product of actinium which calls for 
special mention is actinium A, the direct product of the 
emanation, which, like thorium A, has an extraordinarily 
short period of life. Indeed, actinium A is the most 
unstable element directly known, its period being only 
about ^-girth of a second. It may be put into evidence 
by the same device as that described for thorium A 
(p. 197), but, of course, the endless wire has to be driven 
considerably more rapidly than is necessary to exhibit 


the thorium product. As a matter of fact, a forgotten 
experiment of Giesel, eight years before the discovery 
of actinium A, clearly puts the existence of that sub- 
stance into evidence, when rightly interpreted. If a 
zinc sulphide screen is brought opposite to the open end 
of a tube containing an actinium preparation, and a 
little distance away from it, there is a diffuse luminosity 
produced on the screen in the dark, due to the emana- 
tion escaping from the tube. If the screen is now con- 
nected with the negative pole of an electrical machine, 
instantly there flashes out on the screen a sharply-defined 
bright spot of the same geometrical form as the opening 
of the tube. On discharging the screen this spot in- 
stantly disappears. Giesel thought, very naturally, 
that he was dealing with a new kind of radiation, 
attracted by a negatively charged surface, and called 
the supposed radiation the " E-ray," in brief for " emana- 
tion ray." However, it is not the ray, but the exces- 
sively short-lived product giving an ordinary «-ray, 
which is attracted to the negative surface; but owing to 
the infinitesimal time this product remains in existence 
it appears as if it is the ray, rather than the product, 
which is attracted by the electric field. Another way 
of showing the same experiment is to coat a wire with 
zinc sulphide, and to immerse it in a flask containing 
an actinium preparation. On charging the wire nega- 
tively to the flask, the zinc sulphide instantly flashes 
out and remains brilliantly luminous ; but on discharging 
the wire, the luminosity disappears apparently instan- 
taneously. The same device can be used to show the 
existence of thorium A, but an appreciable, though 
small, time-lag occurs before the appearance and the 
decay of the luminosity. 

Eka-tantalum or Proto-actinium. 

In 1919 the main problem of the origin of actinium 
was cleared up by the discovery and isolation of its 
direct parent in uranium minerals by Cranston and the 


writer in this country, who named it "eka-tantahim," 
and by Otto Hahn and Miss Meitner in Germany, who 
named it "proto-actinium." In each case the search 
was helped by some wide and far-reaching generahsa- 
tions, still to be dealt with, from which it was possible 
to predict the chemical character of the missing parent 
and its place in the periodic table. This place was the 
last and still vacant place in the niobium-tantalum 
family, between uranium on the one side and thorium 
on the other. Mendelejeff, who was one of the dis- 
coverers of the Periodic Law, had called attention to 
three vacant places in the families of boron, aluminium, 
and silicon respectively, which he assumed were occupied 
by three elements still to be discovered, and which he 
called eka-boron, eka-aluminium and eka- silicon. In 
each case he was bold enough to predict their chemical 
character from their position in the table. Shortly 
afterwards the three missing elements were found, and 
named scandium, gaUium, and germanium, and their 
properties were found to correspond very closely with 
what had been predicted. 

In the present case " eka- tantalum," a still unknown 
element, analogous in chemical character to tantalum, 
had been foreseen to be probably the missing parent of 
actinium. Beyond the fact that it has been isolated 
and that it produces actinium slowly and steadily with 
the lapse of time, just as ionium produces radium, not 
much work has yet been done on it. It gives a-rays, 
and from the range of these it is estimated that its period 
is of the order of from ten to a hundred thousand years. 

Uranium Y. 

One more member remains to be considered, and that 
is uranium Y, a radioactive product of short period of 
average life, 2*2 days, discovered by Antonoff in 1911 
to be produced by uranium, and giving (/3)-rays some- 
what more penetrating than those of uranium Xj. It 
is probable that this is the immediate parent of eka- 



tantalum, and the first member of the actinium branch 
series. The branching is thought to occur either at 
uranium I or uranium II, probably the latter, and that 
in both branches an a-iay is expelled. So that 
the initial changes of the series, represented in Fig. 28 
as a single change, has been gradually and with diffi- 
culty traced out to be something as follows: 

^® ■^•(^^ ^•;3r«"rf7; ® © @ 

(238) ^^34) ^ (234) ^(234) &-(23o) ^(226) >- &c. 

Uranium I Uranium X, Uranium X2 Uranium II Ionium Radium 

8,000,000,000 3S-S days 1-65 minutes 3,ooo,ooo\ 100,000 years 2,500 years 

years >l^^»(W._^^® 

^230^ — ^(230) -^^26) — ^ &C. 

Uranium Y Eka Tantalum Actinium 
2-3 days 10,000 to 30 years (?) 

100,000 years (?) 
Fig. 41. 

Considerations to be now dealt with have shown it to 
be of extreme importance that every change in the series 
should be separately and correctly recognised, and when 
this was sufficiently the case, a very great advance 
indeed resulted. 

Thus, with the discovery of these remaining sub- 
stances, the science of radioactivity now embraces 
thirty-six examples of elements in the course of spon- 
taneous change, with periods varying from tens of 
thousands of millions of years on the one hand, to a 
few billionths of a second on the other. It is unlikely 
that any more of these unstable elements remain to be 
discovered, unless some entirely unknown and un- 
suspected source of radioactive materials is found. The 
complete series are set out in detail in the table opposite 
(Fig. 42). 

The Unsolved Riddle of Matter. 

There remains unsolved that most fundamental and 
inaccessible problem, which at the same time appears 
to be the problem of ultimately the most practical signi- 



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ficance — the real internal nature of matter. How is the 
atom of matter put together, and how can it be pulled 
apart ? These are the practical questions which the 
discoveries of radioactivity raise in a pressing form 
without as yet affording a hint of the answer. But in 
spite of that, our knowledge of the internal structure 
of the atom continues to grow at a very rapid rate, and 
some of this more recent work may now be dealt with. 



A Flood of Knowledge. 

Ever since the recognition of radioactivity, the dis- 
covery of radium, the establishment of the theory of 
atomic disintegration, and the independent proof by the 
spectroscope that the element helium is actually being 
produced in a natural transmutation — discoveries 
which followed one another rapidly as the nineteenth 
century passed away and the present century succeeded 
it — it must have seemed to many that such a period of 
pioneering and fundamental reconstruction in science 
would soon exhaust itself and be succeeded by one of 
steady spade-work in the cultivation of the new terri- 
tory opened up. The development of the new territory 
and its detailed exploration have gone on steadily and 
rapidly, but, so far from the wave of original and fertile 
ideas having exhausted itself, the initial successes above 
mentioned have proved to be but the first indications 
of a continuously advancing tide. Already from many 
totally distinct directions the flood of knowledge has 
revealed many of the deeper secrets of the constitution 
of matter. Ignorance and impotence in this field still 
keeps the human race within its traditional boundaries, 
and Nature still holds the final citadel against all 
comers. But now it is being undermined from all sides, 
and changes its aspect almost from day to day, like an 
erstwhile impregnable barrier that is crumbling away 
before our eyes. 

The years 1911-13 witnessed a convergence of 
powerful new methods which, though their simultaneous 



development must be regarded as largely fortuitous, all 
bear definite experimental testimony concerning the 
hidden internal construction of the atom of matter. In 
fact, we can now distinguish therein three distinct 
regions, one within the other, between which probably 
no interchange whatever of constituents occurs, but 
through which, in succession, the atom makes the par- 
ticular impression by which we recognise it in the 
external world, and by which, in turn, it is successively 
guarded from any direct influence from without. The 
first, outermost region is that which the older sciences 
of physics and chemistry have studied so minutely, and 
which is directly concerned in endowing the atom with 
most of those properties by which in the past it has been 
recognised and studied. The second is an intermediate 
region which can be reached and set into the vibration 
known to us as X-rays, by the purely artificially generated 
projectiles, the free-flying electrons or cathode-rays of 
the Crookes tube, dealt with in Chapter IV. The last 
and innermost region of the atom, or the nucleus, has 
never yet been reached save by methods which we owe 
solely to the study of natural radioactive changes and 
by the projectiles, of such inimitable swiftness and 
energy, which are spontaneously expelled during those 

The Nature of Mass. 

Actually before the coming of radioactivity, the dis- 
covery of the electron, a particle more minute than the 
smallest individual atom of matter, had given, in the 
hands of Oliver Heaviside and Sir Joseph Thomson, a 
possible clue to the nature of mass (p. 57). Without 
any direct evidence that the mass of matter was, in 
fact, due to this cause, the reasoning indicated that, if 
the electron were sufficiently small — if the electric 
charge of which it consists were concentrated into the 
volume of a sphere of about 2 x 10"^^ cm. radius, which 
is about one-hundred thousandth of the usually accepted 
value for the radius of an atom — it would possess a mass 


equal to that found — namely xTgo-th part of the mass of 
the hydrogen atom, by virtue of thoroughly well-known 
and understood electro-magnetic principles. A charge 
of pure electricity, entirely unassociated with matter, as 
the negative electron is believed to be, cannot be moved 
from rest without an expenditure of energy, nor if 
moving can it be brought to rest without yielding up its 
energy. It, therefore, must possess inertia or mass. A 
moving charge of electricity is indistinguishable from a 
current of electricity. In the case of an ordinary electric 
current " self-induction " opposes both its starting and 
stopping. If we trace further the origin of the " self- 
induction " in the case of a flow of the electric current, 
or " electro-magnetic inertia " in the case of an indi- 
vidual electron, both terms being technical expressions 
for an identical action, we find it in a fundamental dis- 
tinction between electrostatic and electrodynamic pheno- 
mena — ^that is, between a charge at rest and one in 
motion. The former has no magnetic properties, whereas 
the latter has. The space surrounding a current of elec- 
tricity, or a moving charge, is endowed with magnetic 
properties, and the change in the surrounding space 
when an electric charge, before at rest, is caused to 
move, demands the expenditure of energy. This change 
is believed to be transmitted outward from the electron 
with the velocity of light. This endows a purely electric 
charge with inertia or mass. So that a charge of pure 
electricity must, if sufficiently small and concentrated, 
simulate matter in its most fundamental attribute. 
For the same charge concentrated into spheres of different 
radius, the mass is inversely proportional to the radius. 
Aie there, then, two kinds of inertia or mass, the one 
" material " and the other " electro-magnetic," the one 
for matter, still a fundamental, and the other for elec- 
tricity, a derived conception that can be fully explained 
by the phenomena known to attend its motion ? 


Sir Joseph Thojmson's Model Atom. 

From this the idea arose naturally and was developed 
by Sir Joseph Thomson, that atoms of matter might be 
compounded of electrons in sufficient numbers to account 
for their mass. For each atom nearly 2,000 electrons 
per unit of atomic mass would be required. The prob- 
lem of atomic constitution resolved itself into one of how 
to maintain such systems of electrons in permanently 
stable regime. The early attempts had little of reality 
to recommend them, because by no known means could 
such systems of electrons be held together without 
assuming the existence of positive electricity in some 
form. But positive electricity, existing like negative 
electricity divorced from matter, refused to be dis- 
covered, and, in fact, still remains unknown. In Sir 
Joseph Thomson's model atom, the negative electrons 
were supposed to revolve in orbits within a uniform 
sphere of positive electrification of the same dimensions 
as the atom. It had one very great and suggestive 
merit, for it showed that the electrons would tend to 
arrange themselves in rings. If the number of the 
electrons were steadily increased, the newcomers would 
incorporate themselves into the existing outer ring until 
a certain number had been added, and then, if the 
numbers were further increased, these existing rings 
would become unstable, and the superfluous members 
W^ould at a certain number suddenly rearrange them- 
selves into a new permanent outer ring concentric with 
those previously existing. 

The Periodic Law. 

This simulates very well the known facts with regard 
to the elements as shown by the Periodic Table. Arrang- 
ing the elements in increasing order of atomic mass we 
get the well-known periodicity of chemical properties, the 
tenth element resembling closely the second, the eleventh 
the third, and so on, hydrogen being an exceptional 


element without analogues. So that all the elements 
fall naturally into families, successive members in the 
same family being separated from one another by seven 
intervening elements in the early part of the table, and 
by seventeen in the latter part of the table. The Periodic 
Table is shown in Fig. 43. The elements are set down 
successively in order of increasing atomic weight hori- 
zontally, the vertical columns then contain families of 
chemically allied elements. The separate places are 
numbered consecutively at the top of the place. These 
numbers are the so-called atomic numbers. Below the 
name of each element is its chemical symbol and its 
atomic weight. The families are numbered 0, I, II, 
etc., and these " Group Numbers " express, with certain 
reservations, the usual chemical valency of the element 
— that is, the number of units of affinity with which it 
enters into combination with other elements. Thus, 
aluminium is in the IlIrd family and carbon is in the 
IVth. When these combine it takes four atoms of 
aluminium, each atom with three units of affinity, to 
combine with three of carbon, each with four units of 
affinity, the compound, aluminium carbide, being 
represented by AI4C3. After the IVth group, the ele- 
ments frequently combine to form compounds with 
many different valencies. But here it may be stated, 
though the simple rule is often not followed, that the 
most usual valencies are either the group nimiber or 
eight minus the group number. Thus, nitrogen either has 
five valencies or three, chlorine one or seven, and so on. 
That elements possess units of combining power, or 
" bonds of affinity " as chemists call them, is one of 
the numerous facts which has been, at least partially, 
explained by the discovery of the electron and the fact 
that electricity exists in atoms no less than matter. 

Electrolytic Dissociation. 

During the last quarter of the nineteenth century, 
the theory of electrolytic dissociation, put forward by 



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Svante Arrhenius of Stockholm, became generally estab- 
lished. It asserted that compomids of the class which 
conduct the electric current in the state of solution, and 
known as electrolytes, exist in solution in a more or 
less completely dissociated condition, as oppositely 
charged positive and negative ions, the migration of 
which to the opposite poles constitutes the electric 

It would be idle to pretend that complete clearness of 
interpretation has yet been attained, but the facts 
appear somewhat as follow: The elements, sodium and 
chlorine in Groups I and VII respectively, both act 
usually as elements with a single unit of valency, but 
they belong to, and are typical of, two distinct classes 
of elements. Sodium is a typical basic, metallic, or 
electropositive element, and chlorine is a typical acidic, 
non-metallic, or electro-negative element. They com- 
bine together with the utmost avidity to form common 
salt, NaCl. But in solution in water we are forced, by 
its behaviour to the electric current as an electrolyte, to 
recognise that the complex NaCl does not exist as such, 
at least for the most part. Rather, there are two new 
particles, " sodion " and " chlorion," which exist apart, 
and are called ions. The sodion — Na+ — is an atom of 
sodium carrying one atomic charge of positive elec- 
tricity, and the chlorion — CI" — is an atom of chlorine 
with one atomic charge of negative electricity. It is as 
though the act of chemical combination of metallic 
sodium with the element chlorine was essentially the 
transfer of an electron, or atom of negative electricity, 
from the sodium atom, to the chlorine atom. The 
sodium readily loses a constituent negative electron to 
another element, such as chlorine, which will take it up. 
But although equal numbers of positive and negative 
ions may exist as separate particles when mixed together, 
neither kind can exist alone. The enormous forces of 
electrical repulsion between the similar charges, un- 
neutralised by the presence of the opposite kind, effec- 
tually prevent this being even conceivable. Whether 


an element loses or gains one or more electrons, however, 
is not a self-contained property, but depends on the 
nature of the other element or elements in the com- 
pound formed, so that frequently elements in the 
later families, V, VI, and VII, which may usually 
tend to gain 3, 2, or 1 electrons and to act as acidic 
elements, may act like basic elements and lose 5, 6, or 
7 electrons. 

Undoubtedly, in the broadest sense, though much is 
not yet so clear, the chemical combining power of an 
element is to be explained by the inherent tendency 
the atom possesses either to attract from, or to yield 
up to, another atom, one or more electrons. The act of 
chemical combination, in some of the best-known and 
typical cases, which, in an earlier day, was depicted as 
due to the powerful attraction of one atom for and by 
another, is primarily not exerted between the two atoms, 
but between one of the atoms and the constituent 
electron or electrons of the other. Between the two 
material components of such a stable compound as 
sodium chloride no cohesion or attraction probably 

The molecule of sodium chloride, at least in the liquid 
state, either when fused or dissolved, consists essentially 
of two separate particles or ions, mixed together rather 
than combined, which being oppositely electrically 
charged, must always exist together in equal numbers 
in order that the whole may remain electrically neutral. 
But there is no definite bond of union other than this 
purely electrical requirement, and this refers merely to 
the aggregate number of each kind of particle rather 
than to the individuals. Apart from this limitation, 
the sodium and the chlorine in salt water exist separately 
and totally uncombined for the most part. The in- 
tensity of the electrical charge on an ion is almost incon- 
ceivably greater than any known for matter in the mass. 
It has been calculated that the mutual repulsion be- 
tween the charges carried, for example, by the hydrogen 
ions, would be sufficient to burst the strongest tube that 


can be made, long before there was forced in as much 
hydrogen, in the form of ions, as would, in the ordinary 
state, showthe hydrogen spectrum in a vacuum tube. This 
assumes, of course, what is really quite impossible, that 
such free ions could be put into a tube without being 
discharged by contact with the walls of the tube. 
The " chemical combination " of the partners in a com- 
pound completely dissociated, as sodium chloride is in 
liquid form, is due to a purely electrical and statistical 
partnership of the otherwise completely independent 
ions, which, in the modern view, is practically as effec- 
tive in maintaining the combination as the rigid bonds 
linking each individual sodium atom to one chlorine 
atom which Dalton first pictured. This refers to the 
class of electrolytically dissociated substances, which 
comprises the acids, bases, and salts, and not to the very 
large class of non- electrolytes, which comprises all the 
organic compounds, where permanent individual unions 
between the atoms of the molecule undoubtedly exist. 

The Outermost Region of the Atom. 

Chemical changes and chemical properties, in general, 
deal only with the outermost region of the atomic 
structure, and we shall not probably do violence to the 
facts if, without at present attempting to review all 
the evidence for this conclusion, we picture it as con- 
taining a certain number of " valency " electrons. 
This number is the same for all the members in the same 
family or vertical row of the periodic table, and differs, 
literally, unit by unit in passing hoiizontally from one 
family to the next. For a certain number of electrons 
in the outer ring — namely, that possessed by the zero 
family — there is no tendency for the atom either to lose 
or gain electrons. The members of this family, which 
comprises the inert gases of the atmosphere, are totally 
devoid of chemical affinity. The next family, in 
Group I, which contains the alkali metals, has one 


electron more than this number, which is relatively 
loosely held. In all probability it moves in an orbit far 
external to all the rest. In the other direction, in 
Group VII, containing the halogen family, the number is 
one less than this number, and these elements readily 
take up an additional electron in the presence of an 
element of Group I which has such an electron in excess. 
The outer ring of electrons seems for all atoms to try and 
conform to a certain standard number. Atoms with 
less rob the ones with more, and this process probably 
constitutes, in the main, chemical combination. Whether 
the robber and the robbed entirely part company, as in 
the electrolytes, or remain interlocked, as in organic 
compounds, is a secondary consideration. We may 
suppose that, when the number of electrons in the outer 
ring exceeds a certain limit, which in the first part of the 
periodic table is seven, a complete new inner ring of 
eight electrons is formed. The chemical properties, 
however, depend only on the outermost ring directly, 
and the inner rings exert a subordinate effect. The 
valency of such an element and its general chemical 
nature resembles, therefore, the eighth preceding 
element. This holds in the early part of the periodic 
table. At the 22nd element, titanium, a new and more 
complicated dual periodicity commences, in which the 
number of elements separating the consecutive members 
of one family is eighteen instead of eight. A new group 
of three closely allied elements, the so-called Vlllth 
Group, now appears in the middle of the period, where 
previously an argon element would appear, and the 
next seven elements have a partial analogy to the seven 
preceding the Vlllth Group. The easiest way of 
regarding the matter is to suppose that ten metallic 
elements, indicated in Fig. 43 between { } are inter- 
polated into the old short periods. 

At the 57th element, lanthanum, the law suddenly 
and completely breaks down. A group of seventeen 
elements, known as the rare-earth elements, and of which 
two remain to be discovered, is interpolated into the 


series at this point. They all resemble one another and 
lanthanum with such extreme closeness that their separa- 
tion and identification is one of the most laborious and 
difficult tasks that the chemist can undertake. At 
tantalum, the 73rd element, the series begins again 
almost as if it had not been interrupted, and continues 
normally to the end. 



The Innermost Region of the Atom. 

Now let us see what radioactivity can tell us of the 
insides of these atoms, for be it remembered that though 
the older chemical and physical properties of matter are 
concerned only with the outermost shell, the seat of 
government which impresses upon any atom its chemical 
character, and which conditions that chlorine should 
resemble bromine and differ from potassium, is inside 
the atom, in a region impenetrable to the methods of 
investigation known at the opening of the century. 
From such methods we could only guess what might be 
inside, and the guesses never even approached the truth. 
But now we can send a messenger right through the 
unknown territory, which perchance may, on re- 
emergence, tell us something of more interest and value 
to the race than any traveller who has ever struggled 
back again into being from the waste places of the eartfi. 
And this messenger, whose speed must be comparable 
with that of light and whose mass must be comparable 
with that of the atom it is to invade, is the a-particle 
(Chapter IV.). We owe to the genius of Sir Ernest 
Rutherford the recognition of the importance of this 
new method of attacking the most fundamental of all 
problems, that of the ultimate structure and constitution 
of the atom. Together with his students, he has made 
a close quantitative study of the effect on the os-particle 
of its passage through the various atoms of matter. 
Though, as Bragg showed, the a-particles pass straight 



through the atoms, this is not the whole truth. Thou- 
sands of a-particles pass through thousands of atoms in 
their path, almost as if they were not there, suffering but 
slight retardation and hardly any appreciable deviation 
from their course at each encounter. But there occur 
also, and as an exception, large deflections (compare 
Fig. 19), and occasionally the a-particle is violently de- 
flected through a large angle by an exceptionally close 
encounter, like a comet passing round the sun. It may 
even emerge from the side it entered. This is termed 
" occasional large-angle scattering " to distinguish it 
from the incessant very slight deviations, first in one 
direction then in another, according to the laws of 
probability, which, as a more minute examination has 
shown, is continually happening to the a-particle as it 
ploughs its way through the atoms. Inevitably this 
makes us view the atom itself as consisting essentially 
of a very small dense nucleus at the centre of a relatively 
enormous and almost empty sphere of influence con- 
taining only electrons. The a-particles, being immensely 
more massive than the electrons, are not seriously dis- 
turbed by the rings or shells of electrons whose revolu- 
tions determine the apparent size of the atom as fixed 
by the older physical methods. Against all other 
invaders, these swiftly revolving satellites guard the 
interior of the atoms as efficiently as if the atom really 
occupies the space to the exclusion of everything else. 
But such exclusive occupation of a definite volume of 
space by matter is an illusion. A material projectile, 
like the a-particle, moving at a speed the tenth of that 
of light, passes through all the electronic ring-systems 
as an errant sun might pass through the solar system. 
This happens many thousands of times without any 
serious consequences to the a-particles, or to the atomic 
system invaded. But an occasional a-particle finds its 
mark, and heads straight for the real atom — that is to 
say, the central nucleus in which the material as dis- 
tinguished from the electrical constituents of the atom 
are concentrated. 



The a-particle we know to be a helium atom of mass 4 
carrying two atomic charges of positive electricity. 
Or, more accurately, a helium atom is an a-particle 
minus two electrons. In all probability the a-particle 
is the simple nucleus of a helium atom, the central sun, 
as it were, alone and unattended by any electronic 
satellites or planets at all. The size of this central 
nucleus of the atom, in relation to the apparent size 
of the atom, is probably of the same order of magnitude 
as that of the earth to the whole solar system. Ruther- 
ford, for example, from these experiments, considers 
that practically the whole mass of the hydrogen or 
helium atom is contained in a central nucleus of dia- 
meter one hundred thousand times smaller than the 
accepted diameter of the atom. This central nucleus 
carries a positive charge, to the extent of about one 
unit, or atomic charge, of positive electricity, for every 
two units of atomic mass. For each unit of positive 
electricity resident in the nucleus a similar unit of 
negative electricity, or an electron, revolves in one or 
other of the outer shells, so that the negative charge on 
the electronic systems is neutralised by the positive 
charge on the nucleus or material core. This model of 
Rutherford's differs essentially from the earlier models 
in that it has been based on a careful and exhaustive 
experimental examination of the single large-angle 
scattering of a-particles. 

Now let us consider the exceptionally close encounters, 
when nucleus meets nucleus and large-angle deflection 
of the a-particle results. If the atom invaded by the 
a-particle is massive by comparison, the positively 
charged nucleus constituting the a-particle will be 
violently repelled as it impinges on the very much more 
intensely positively charged and much more massive 
nucleus of the heavy atom, and will be violently swung 
out of its path, much as a comet is at perihelion. It is 
true that the forces at work are repulsive rather than 
attractive, but this makes no essential difference. If the 
two nuclei happened to meet absolutely " head-on " the 


a-particle would be repelled the way it came almost at its 
original velocity. 

But when a-particles traverse atoms lighter than 
themselves — for example, atoms of the gas hydrogen — a 
different state of things must obtain. Here an abso- 
lutely " head-on " collision would result in the hydrogen 
atom being repelled in the same direction as that in 
which the a-partlcle was travelling, but with a velocity 
far in excess of that of the original «-particle. In fact, 
this hydrogen atom will then behave as a new kind of 
radiant particle, and by virtue of its smaller mass and 
charge and greater velocity it should travel through the 
hydrogen gas far further than the original «-particles 
before being stopped. Marsden has shown that when 
the a-particles are made to pass through hydrogen and 
their range examined by means of a zinc sulphide 
screen, in addition to the scintillations given by the 
a-particles themselves, a few weaker scintillations, which 
must be due to the repelled hydrogen atoms, can be 
observed at distances from the source some four times 
greater than the a-particles themselves are able to pene- 
trate. These new particles may be termed " H-particles " 
for the sake of clearness. 

An Artificial Transmutation. 

In 1919, by the work of Sir Ernest Rutherford, a 
further important step in this advance was taken, which 
raises the question whether a beginning has not already 
been made in the achievement of actual artificial trans- 
mutation to an infinitesimal extent. It has been recog- 
nised, by the late Sir William Ramsay among others, 
that, of all known agencies likely to be able to transmute 
one element into another, the a-particle, on account of 
its unique kinetic energy, was the most likely to prove 
effective. The work described shows how exceedingly 
difficult it is to hit the real atom exactly with the 
a-particle. Later results have proved that only about 
one out of 100,000 a-particles, in passing through one 


centimetre of hydrogen gas at normal temperature and 
pressure, produced an H-particle. Since, in this path, 
the number of hydrogen atoms penetrated is 10,000, in 
only one out of one thousand million collisions is the 
nucleus of the atom of hydrogen really hit. In the rare 
case when the a-particle actually impinges upon the 
nucleus, it is to be anticipated that the latter, if not 
an exceedingly stable system, might sometimes be 
broken up. 

Of the common gases, hydrogen, oxygen, carbon 
dioxide, and nitrogen, which he exposed to the bom- 
bardment of the a-particles, Rutherford observed an 
anomaly in the case of nitrogen. These gases all gave 
the expected effects, namely, the production of " N- 
particles " and " 0-particles " — that is to say, new 
rays were observed, longer in range than the a-rays, 
which were first thought to be atoms of these ele- 
ments with a single positive charge put into violent 
motion by collisions of the a-particles with the nuclei 
of the oxygen and nitrogen atoms, always in the 
minute numbers to be expected from the results with 
hydrogen. In these cases the range of the new particle 
is only slightly longer than that of the a-particles them- 
selves. But in nitrogen there were observed, in addi- 
tion, particles of the long-range and other characteristics 
exactly similar to the H-particles produced in hydrogen 
gas. Only one such H-particle was observed for every 
twelve N-particles produced. These results strongly 
suggest, though they do not yet rigorously prove, that 
the nucleus of the nitrogen atom struck by an a-particle 
is occasionally shattered by the collision, and that 
hydrogen atoms are produced from it. It may be sur- 
mised, for example, as one possibility, that the nitrogen 
atom of mass 14 is converted into a carbon atom of 
mass 12 and two hydrogen atoms. The excessively small 
proportion of the nitrogen atoms penetrated by the 
a-particles, which are so shattered, must not be for- 
gotten. This makes it exceedingly unlikely that such 
a case of artificial transmutation, if it occurs, can ever 


be directly confirmed by direct chemical analysis. It 
must also be remembered that in this case, even if it is 
correctly interpreted, transmutation has not been really 
artificially initiated. What has been done, at the most, 
is to use a naturally occurring transmutation, that can 
still be neither initiated artificially nor controlled, to 
produce a secondary transmutation. The real problem 
of how artificially to transmute one element into an- 
other at will remains still completely unsolved. 

While this book was passing through the press, 
Ruth erf Old has published further results, in which 
the real nature of these particles, generated by the im- 
pact of the a-rays in different gases, has been examined 
by the method by which the nature of the electron and 
the a- and ^-particles has been established — that is 
to say, the particles were subjected to the action of 
electric and magnetic deviating fields, and, from the 
magnitude of the deflection, the mass, the charge, and 
the velocity were determined. This established the 
correctness of the earlier conclusion that the H-particles 
generated in hydrogen, and also in nitrogen, consisted 
of singly positively charged hydrogen atoms. But it 
was found that what have been termed " N-particles " 
and " 0-particles " were not singly charged atoms of 
nitrogen and oxygen, as first surmised, but for each the 
same and an entirely new particle of mass 3, carrying 
two positive charges. On the views discussed in the next 
chapter they would appear to be atoms of an isotopic 
variety of helium, otherwise unknown. 

Thus the new results confirm the conclusion that the 
nitrogen atom is shattered during a close nuclear 
collision with an «-particle, but it appears to suffer 
disruption in two independent ways, giving, in one way, 
atoms of hydrogen of mass 1, and, in the other, atoms 
of a new kind, of mass 8. In the case of the oxygen 
atom the latter particles alone appear to be produced 
(Sir Ernest Rutherford, Bakerian Lecture, Royal 
Society, June 3, 1920). 


Atoms compared and contrasted with Solar 

Thus, inevitably as science proceeds, the solid tangible 
material universe dissolves before its touch into finer 
and still finer particles, the unit quantities or " atoms " 
of positive and negative electricity. The passive attri- 
butes of matter in occupying a definite volume of space 
to the exclusion of other matter resolves itself into an 
active dynamic occupation by virtue of the sweep of 
the electronic satellites in their orbits round the positive 
central sun. But whereas, in the solar system in which 
we live, the central sun is both large and massive in 
relation to the sizes and masses of its attendant planets, 
in the atomic solar systems there is a curious inversion. 
From the facts disclosed in reference to the passage of 
a-particles through hydrogen, it would appear that 
the centres of the two colliding nuclei, the hydrogen 
nucleus and the helium nucleus, approach to within 
a distance of less than the accepted diameter of the 
negative electron. The central material nucleus, in 
which all but a negligible part of the mass of the atom 
is concentrated, thus appears to be at least as small as, 
and probably smaller than, the negative electron, the 
smallest particle previously known to science. Since 
the smaller the volume in which a given electric charge 
is concentrated the greater will be its mass, it may really 
be that the positive electron is very much more concen- 
trated and very much more massive than the electron, 
and that the nucleus of the hydrogen atom, the sim- 
plest of all atoms, is in reality the missing positive 
electron. But this, at present, is merely a suggestion. 
The positive charge is the same in amount as the nega- 
tive charge of the electron. For its mass to be that of 
the hydrogen atom, which is 1,830 times that of the elec- 
tron, its radius must be 1,830 times less, or about 10~^^ cm. 



Elements which are Chemically Identical. 

In another totally distinct direction, radioactivity has 
been the means of throwing a flood of light on the nature 
of matter and in particular on the periodic law of the 
elements, which epitomises the existing chemical know- 
ledge of matter. In the first chapter, the underlying 
limitations which attend all knowledge were empha- 
sised. Such an underlying limitation is revealed by the 
sequence of radioactive changes. In Chapter X. (p. 154) 
it was shown that many of the known radioelements 
resemble others so completely in their chemical nature 
that no separation can be effected once they have been 
mixed, and in Chapter XII. we came upon numerous 
further examples of the same resemblance among the 
members of the thorium disintegration series. No 
chemist could detect by chemical analysis the separate 
existence of the two uraniums, uranium I and II, or of 
thorium and radiothorium, or mesothorium and radium, 
or of lead and radiolead, in a mixture containing any of 
these pairs. Naturally the question was asked whether 
any of the common elements, for which radioactive 
methods of analysis are not available, are, as supposed, 
really homogeneous elements, and whether any are 
mixtures of different elements, with different atomic 
weights, but with identical chemical properties, so 
merely appearing to be homogeneous to chemical 
analysis. Matter is, in all probability, far more com- 
plex than chemical analysis alone is able to reveal, 
because radioactivity has shown us the existence 



of elements identical in their chemical behaviour, 
but, nevertheless, distinct in atomic weight and in 

The Periodic Law and Radioactive Changes. 

In 1911 the writer pointed out that the products of 
«-ray changes have a certain definite relationship in 
chemical character to their parents. The chemical pro- 
perties of an a-ray product correspond with those of an 
element in the periodic table with group number two 
less than that of the parent. Thus, radium in Group II 
expels an a-particle and changes into the emanation in 
Group 0, ionium in Group IV changes by expulsion of an 
«-particle into radium in Group II, and so on. It was 
also noticed that the passage through the periodic table 
of the element undergoing change was frequently alter- 
nating, the products frequently reverting in chemical 
nature to that of an earlier parent. So radiothorium 
resembles thorium, thorium X mesothorium I, and so 
on. This curious atavism has now been very simply 
and fully explained, largely owing to the chemical in- 
vestigations of Alexander Fleck in the writer's labora- 
tory at Glasgow, who spent three years in the exhaustive 
study of the chemical nature of all the radioactive 
elements, which survive for a long enough period for 
their chemical nature to be determined, and many of 
which had previously been very imperfectly investigated 
from this standpoint. In consequence, the generalisa- 
tion already alluded to in preceding chapters has come 
to light. It was seen that the expulsion of a y8-particle 
was entirely analogous to that of the expulsion of the 
«-particle, but that, instead of the product possessing a 
chemical nature corresponding with an element in the 
periodic table with group number two less than the 
parent, it corresponded with an element of group 
number one greater. Hence if, in any order, one a- 
and two /3-rays are expelled, the product is chemically 
of the same nature as its parent, and the curious atavism 


referred to above is explained. Radioactive children 
frequently resemble their great-grandparents with such 
complete fidelity that no known means of separating 
them by chemical analysis exists. But, of course, the 
two intermediate parents are readily separated. By 
this means all the members of the family may be 
recognised severally, although, but for this means, that 
would be still impossible. 

The complete generalisation, which was put forward 
in 1913 independently during the same month by A. S. 
Russell, K. Fajans, and the writer, is illustrated by 
Fig. 44. The last twelve places of the periodic table, 
from uranium to thallium, are placed consecutively side 
by side, and the passage of the elements, in the uranium, 
thorium, and actinium series, from place to place, as the 
a- and /S-ray changes succeed one another, is indicated 
by arrows. The figure is to be read at 45°, so that the 
lines showing the atomic weights are horizontal. 

Every detail of the chemical nature of the members 
of the known sequences in the uranium, thorium, and 
actinium series, including the complicated branchings 
which occur towards the ends, bears out implicitly 
these two simple rules. Independently of their origin, 
atomic weights, and radioactive character — that is, of 
the kinds of change they are about to undergo — all the 
members of the three disintegration series, which, by 
the consistent application of these rules fall into the 
same place in the periodic table, are chemically com- 
pletely identical and non- separable from one another. 
Hence I have termed them isotopes or isotopic elements. 

The Atomic Numbee. 

Confining attention to the most generally important 
consequences of this embracing generalisation, we may 
at once connect the rules with the fact that the a-particle 
carries a double positive atomic charge and the /3- 
particle a single negative atomic charge. Each of the 
successive places in the periodic table thus corresponds 



Sequence of Changes of Uranium (U) and Thorium (Th) into various 

Isotopes of Lead (Pb). 

Fig. 44. 


with unit difference of charge in the constitution of the 
atom. This suggestion was made tentatively by van 
der Broeck before it was first proved by these researches. 

The discovery of the atomic nucleus by Rutherford 
enables us to go further. It is hardly possible to doubt 
that both the a- and the /S-particles are expelled from 
the nucleus. Hence this difference of charge in the 
constitution of the atom in passing from one place in 
the periodic table to the next must be a unit difference 
in the net positive charge of the nucleus of the atom, 
and a corresponding unit difference in the number of 
negative electrons external to the nucleus, which com- 
pensates the positive nuclear charge and renders the 
whole atom neutral. 

In his original theory Rutherford concluded that the 
magnitude of the positive charge of the nucleus was 
approximately one-half of the number representing the 
atomic weight of the elements. Now, from evidence 
still to be considered, it is known exactly to be equal 
to the number of the element in order of sequence in 
the periodic table, when the elements are arranged in 
order of atomic weight. This number is now always 
called " the atomic number." Usually it is rather less 
than one-half the atomic weight. Uranium, the last 
element, occupies the 92nd place in the periodic table; 
its atomic number is therefore 92, and its atomic weight 
is 238. 

So far as is known the atomic number of hydrogen is 
one, that of helium is two, of lithium three, and so on 
until we arrive at uranium, ninety-two. Gold is the 
79th element, mercury the 80th, thallium the 81st, lead 
the 82nd, and thenceforward, as shown in Figs. 43 and 
44, by the numbers at the head of each place of the 
periodic table. 

IsoTOPic Elements. 

The generalisation proves definitely that, as regards 
the last twelve places in the periodic table, between 
uranium and thallium, the successive places correspond 


with unit difference of nuclear charge and unit differ- 
ence in the number of external electrons as was pre- 
viously assumed. But it also shows that in the ten 
occupied places each place accommodates on the average 
no less than four distinct elements. The atomic masses 
of the various elements occupying the same place vary 
in some cases by as much as eight units, and there is 
nothing to show that the same may not occur through- 
out the whole periodic table. Such groups of isotopic 
elements, occupying the same place, possessing the same 
net nuclear positive charge and the same number of 
electrons in their external systems, are not merely 
chemically identical and indistinguishable. Many of 
their commoner purely physical characteristics, such as 
spectrum and volatility, have also been found to be 

The existence of such isotopic elements would not 
have been suspected except for radioactive changes. 
What fixes the chemical and general material character 
of an element is a particular numerical charge, and this 
charge is not the total charge of the atom, not even the 
total charge of the nucleus of the atom, but is the net 
charge of the nucleus or the difference between the 
numbers of positive and negative charges which it con- 
tains. The same net charge may be, and, in the case of 
isotopes, is made up of different absolute numbers of posi- 
tive and negative charges differing by the same amount. 
When an a- and two y8-particles are successively expelled 
the net charge becomes again what it first was, and the 
position in the periodic table and whole chemical 
character also reverts to the initial state. But the atomic 
mass is different by four units, the mass of the a-particle 

The Problem of the Ancient Alchemist. 

There is one interesting point that may be referred 
to, which serves to show how nearly science has ap- 
proached to the ancient alchemical problem of turning 


base metals into gold. In these spontaneous changes, 
if either actinium D or thorium D had elected to expel 
an a- instead of a /3-par»ticle, the product would have 
been an isotope of gold instead of lead. 

Gold occupies a position in the periodic table two 
places removed from and before thallium, so that if 
thallium could be induced to part with an «-particle, 
the product would be an isotope of gold. If it was 
sufficiently stable it would be gold for all practical 
purposes. It is true its atomic weight and density- 
would be somewhat greater, but otherwise it would be 
the same. Or, again, if bismuth could be made to expel 
two a-particles, or lead an a- and a /3-particle, gold again 
would be the product. This, then, is a list of recipes 
for the modern alchemist, one and all indubitable, but 
one and all awaiting ^ means of accomplishment. It 
remains for the future to show how the nucleus of an 
atom can be so influenced as to be caused to eject an 
a- or /S-particle at will. But it is a tremendous step 
gained to know for the first time in what transmutation 
really consists. 



The X-Ray Spectra of the Elements. 

We now have to turn to yet another great advance. 
Beginning with the case of ordinary light, it is well 
known that it may be analysed into its component wave- 
lengths by the use of a " diffraction grating," as well as 
by an ordinary prism. 

In the Rowland diffraction grating some large known 
number, usually from ten to twenty thousand lines per 
inch, are accurately ruled by a diamond mounted on a 
dividing engine, upon a plane or concave surface of glass 
in such a manner that all the lines are exactly parallel 
and all precisely equally spaced apart. The light trans- 
mitted by such a grating is split up into a large number 
of parallel beams which " interfere " with one another, 
and the result is that the direct beam is more or less 
extinguished, but each different wave-length of light in 
the beam is bent, or diffracted, from its course through 
a definite angle which is different for each different 
wave-length. So the light is resolved, or spread out, 
into a pure spectrum much as when it passes through a 
prism. Now, if the distance between the rulings — one-ten- 
thousandth of an inch, for example — is exactly known, 
the actual wave-length of each line in the spectrum may 
be easily and exactly calculated. A beam of X-rays, 
as we now definitely know, consists of a radiation of 
precisely the same kind as light, but of wave-length 
some ten thousand times shorter. Hence, to resolve it, 
we would require the use of a " grating " at least a 
thousand or ten thousand times more finely ruled thin 



can be ruled by the most perfect dividing engine. Who 
could make such a grating ? 

But an infinitely more perfectly executed, and ten 
thousand times more closely packed, assemblage than 
the finest and most perfect Rowland grating ever made 
was found in 1912, by Laue, Friedrich, and Knipping, 
who discovered that the X-rays are regularly diffracted, 
like light is by the grating, when reflected from the 
surface of an ordinary crystal, such as rock salt, fluor 
spar, calcite, and the like. 

In this country the discovery was eagerly taken up, 
and we owe to the Professors Bragg, father and son, a 
clear insight into the whole subject. In the crystal, as 
the crystallographers have, with eyes of faith, long 
lepicted, the atoms of the substance are marshalled in 
a definite space-lattice' of regular geometric form, so 
that each atom is fixed at a definite point in space at a 
definite distance from and in a definite angular direction 
to all the atoms surrounding it. The smallest number 
of atoms required completely to represent the pattern 
— so that the whole structure is made up simply by 
redupHcating this unit indefinitely in the three dimen- 
sions — is called the space-lattice of the crystal. More- 
over, the distances between the atoms, or points, of the 
space-lattice is, for common crystals, just of the right 
order of length to resolve the X-rays in a manner pre- 
cisely analogous to that in which light is resolved by the 
Rowland diffraction grating. If we know for any one 
crystal what this distance actually is, we can determine 
the wave-length of any X-ray from the angle at which 
it is reflected from the crystal. 

For the ordinary heterogeneous beam of X-rays 
given by an ordinary X-ray tube, which corresponds 
to white light, the beam is resolved by the crystal 
into an X-ray spectrum, and the wave-length of 
the component radiations may be found. If we 
know the wave-length for any one X-ray, we can 
find out for any crystal, in any plane or face we 
choose, the precise distance apart between the atoms 


that make it up, and so we can construct its space- 

This has given crystallographers a powerful direct 
method of testing the reality of the space-lattices which 
have been arrived at by theoretical reasoning and the 
power of second sight of the mathematical mind. The 
results already have been gratifying and remarkable. 
The actual spacial arrangements of the individual atoms 
that go to make up the crystal are now being precisely 
measured and explored, and, as has so frequently hap- 
pened before, the patient theoretical conceptions of a 
generation less brilliantly equipped with experimental 
methods of inquiry are being triumphantly vindicated. 

But it is not with this field we are now most closely 
concerned. It is rather with the wave-length of the 
X-rays, and with their period or frequency, which can 
be so found by this method. If we consider the unit 
of time, one second, in this period light or X-rays travel 
Sxio^*^ cms., whatever the wave-length. But in the 
3 X 10^° cms. of length, or second of time, there will be 
about twice as many separate waves of violet light as of 
red light, and many thousand times more waves of any 
X-ray than of either. From the wave-length we can at 
once find the frequency or " pitch " of the radiation, 
or the number of vibrations per second to which it 
corresponds. This frequency again, in atomic solar 
systems, corresponds with the number of revolutions 
made per second by the electron in its orbit within the 
atom, and this depends on the diameter of its orbit. 
In much the same way we might speak of the frequency 
of a planet as the number of revolutions it makes round 
the sun in a century, and this depends on the distance of 
the planet from the sun. The rays that constitute the 
ordinary visible spectrum arise probably from the 
outermost electrons of the atom, the ones, that is, that 
are responsible for the chemical character and which 
traverse orbits of diameter of the order of 10"^ cm., 
which is the diameter of the atom, meaning by that the 
whole atomic system. To get waves of a thousand to 


ten thousand times shorter length, and frequencies a 
thousand to ten thousand times greater than for visible 
light — to get X-rays, in fact — it is clear that we have 
to get much nearer to the centre of the atom, into a 
region intermediate between that in which the ordinary 
phenomena of physics and chemistry originate and the 
innermost nucleus disclosed by radioactivity. 

The 7-Rays. 

By the same method of reflection from crystal sur- 
faces some, at least, of the 7-rays have also been resolved 
and shown to be X-rays, but of very much shorter wave- 
length in general than those artificially produced. The 
wave-length of light is usually expressed in Angstrom 
units (written A). One Angstrom unit is equal to 
10"^ cm. The wave-lengths of visible light waves vary 
from 6,000 or 8,000 (A) in the red to 3,500 (A) in the 
violet, and to 2,000 in the extreme ultraviolet. ^The 
wave-length of the X-rays range from, perhaps, 8 A for 
very soft X-rays to 0-5 A for the most penetrating type 
that can be produced. But the wave-length of 7-rays 
is in general much less ranging from 1-2 A to as little as 
0-07. Moreover, it is believed that for the most typical 
very penetrating 7-rays of radium and thorium the 
wave-length is far too short even for the crystal to be 
capable of resolving them, and they may have wave- 
lengths 100 times shorter than the shortest yet resolved. 

The existence of rays so short in wave-length and high 
in frequency points to a revolution of electrons in the 
atom in orbits of excessively minute diameter, so 
minute that the question arises whether the 7-rays do 
not really originate from electrons actually contained 
within the atomic nucleus. These results furnish 
another and independent proof that radioactive pheno- 
mena occur entirely in the atomic nucleus. 



The Intermediate Region of the Atomic Struc- 
ture. — The Homogeneous Characteristic 
X-Rays of Barkla. 

A Rontgen tube gives X-rays of all wave-lengths 
within limits which depend on a variety of conditions, 
such as the nature of the metal constituting the anti- 
cathode, the degree of vacuum, and the potential differ- 
ence between the electrodes. The very important dis- 
covery was made by Barkla that, when such X-rays 
impinge upon various metals, they will, if penetrating 
enough, produce new secondary homogeneous X-radia- 
tion, the properties of which are characteristic of the 
metal and not of the primary radiation. Each element, 
except those of less atomic weight than sodium, emits 
under such circumstances an X-ray of definite and 
characteristic spectrum, which differs from the ordinary 
light spectrum given by the same element in being ex- 
cessively simple. Often it consists of a single strong 
line together with one or more weaker ones. Such 
characteristic X-rays belong to various series, designated 
the K-, L-, M-series connected in the following way: 
Beginning with sodium, the 11th element in the Periodic 
Table, the X-ray, characteristic of the element sodium, 
belongs to the so-called K-series, and is extremely 
feebly penetrating and of long wave-length, as the wave- 
lengths of X-rays go. Going up through the elements 
in increasing order of atomic weight, as far as tin, the 
50th element, the K-radiation produced steadily dimin- 
ishes in wave-length and increases in penetrating power, 
until, at tin, it is difficult artificially to generate a 
primary X-ray of sufficient penetrating power to excite 
the characteristic radiation. Hence this experimental 
limitation prevents this series being studied for elements 
of greater atomic weight. 

Before this, however, beginning with the element zinc, 
the 30th element, in addition to the K-radiation, a new 
characteristic radiation of very feeble penetrating power. 


belonging to the so-called L-series, makes its appearance. 
From zinc onward this new radiation increases in pene- 
trating power and decreases in wave-length until the 
last element uranium is reached. Again, at gold, the 
79th element, another new series, the M-series, is first 
observed, very non-penetrating at first, but increasing 
in penetrating power to uranium. 

Moseley made a systematic determination of all the 
wave-lengths of the principal lines of these characteristic 
X-rays from aluminium to silver in the K-series, and 
from zirconium to gold in the L-series, and discovered that 
they are connected together by a simple mathematical 
relation, involving the atomic number of the element. 
The square-root of the frequency (as we have seen the 
frequency is proportional to the reciprocal of the wave- 
length) is proportional to a number that increases by 
one in passing from any element in the periodic table to 
the next. In other words, the square root of the fre- 
quency is proportional to a number that increases in the 
same way as what we have termed the atomic number 
of the elements, when arranged in order according to 
the Periodic law. 

The practical value of this discovery was great. For 
the first time it was possible to call the roll of the 
chemical elements and to determine how many there 
were and how many remained to be discovered. There 
are between hydrogen and uranium ninety-two possible 
elements, of which only six remain to be found — 
namely, the two unknown heavier analogues of the 
element manganese, two rare-earth elements, and the 
two heaviest analogues of iodine and caesium re- 
spectively (see Fig. 43). 

It is curious that the first two should still and for so 
long elude discovery. They would in all probability be 
most useful metals, allied to the noble metals in char- 
acter, the first to the light platinum metals, ruthenium, 
rhodium, and palladium, and the second to the heavy 
platinum metals, osmium, iridium, and platinum. 

As is well known, the Periodic Table comprises certain 


exceptions. Tellurium has an atomic weight higher than 
iodine, though in the periodic table it precedes it, and 
the same is true for argon and potassium, and for cobalt 
and nickel. The X-ray spectra of these elements con- 
firms the order in which they have been put by chemists 
in the periodic table on account of their chemical char- 
acter and despite their atomic weights. This shows 
that it is the atomic number — i.e., the net positive 
nuclear charge of the element, or the number of elec- 
trons external to the nucleus — which fixes the position in 
the periodic table, rather than, as hitherto supposed, 
the atomic weight. The existence of isotopic elements 
of identical chemical character but different atomic 
weight points to the same conclusion. In fact, this 
work on X-ray spectra dovetails perfectly into the con- 
clusions reached, independently, in the study of radio- 
active change, and extends them to all the elements in 
the periodic table. 

The Atomic Mass or Weight. 

The chemical character, and even the spectrum of an 
element, at least to a degree of approximation attainable 
by common methods, depends upon the atomic shell 
and not upon the atomic nucleus, and the character 
of the shell is identical, whatever the nucleus, so long 
as the atomic number is the same. The atomic mass or 
weight, on the other hand, on the views adopted, is to 
all intents and purposes a property of the nucleus alone. 
Mass and radioactivity, the oldest and the newest pro- 
perties of matter, are in this respect allied and sharply 
to be distinguished from all the other properties. Iso- 
topes have in general nuclei of different mass but the 
same net positive charge, and therefore their outer 
electronic systems and all the properties which origin- 
ate therein — that is to say, all properties save mass 
and radioactivity — are practically identical and indis- 

We have seen that radioactive change afforded a very 


subtle way of separately distinguishing between and of 
actually separating isotopes in favourable cases. In the 
disintegration sequence A>B^C>D>, A, B, C are neces- 
sarily elements completely distinct chemically and 
capable of easy separation by chemical analysis. But 
if in the three changes, one a- and two yS-particles are 
expelled, D is necessarily chemically identical with A, 
but of atomic mass four units less. 

Because of the change, D can be apprehended as an 
individual, and, since B and C are separable from A and 
in course of time turn into D, in cases when the periods 
are favourable, D can be separated from A. Except for 
the change, A and D would, in spite of the difference in 
their atomic weights, be mistaken by chemists, relying 
on the usual chemical and spectroscopic criteria of purity 
and homogeneity, for a single homogeneous element. 
Its atomic weight would be a mean of the atomic weights 
of its constituents, depending not only on the magni- 
tude of each, but on the proportions in which they were 
mixed. This would apply not merely to the radio- 
elements but equally to all. It is therefore, perhaps, 
not altogether surprising that all the many efforts made 
to find exact numerical relations between the atomic 
weights of the various elements should have proved 

The Element Lead. 

These ideas have been put sharply to experimental 
test in the case of the element lead. As the generalisa- 
tion illustrated by Fig. 44 shows at once, the ultimate 
products of all the disintegration series in all branches, 
so far as they have been traced, end in the same place in 
the periodic table — namely, the place occupied by lead. 
Therefore, in spite of the differences of origin and of 
atomic weight, they must all be isotopes of lead, if the 
apparent ends of the series coincide with the actual ends 
and no further, as yet undetected, changes occur. 

The atomic weight of ordinary lead is 207-2, whereas 
that of the main branch of the uranium series, is 206, 


and that of both the branches of the thorium series is 
208. The atomic weight of the end product of the 
actinium branch series is doubtful, but, as it is only 
present in small relative quantity, it may be, in the first 
place, neglected. Clearly, if this view is correct, the 
lead derived from a uranium mineral ought to have an 
atomic weight somewhat lower than that of ordinary 
lead, and the lead derived from a thorium mineral an 
atomic weight somewhat higher. The prediction, like 
so many that have been made in this subject, has been 
completely confirmed by experiment. Lead from care- 
fully selected uranium minerals, not containing thorium 
in detectable quantity, has been found to have an atomic 
weight as low as 206-05. Lead from carefully selected 
thorium minerals containing only a small quantity of 
uranium has been found to have an atomic weight as 
high as 207-9. Chemically, they are identical and in- 
distinguishable from common lead, which, indeed, may 
well be a mixture of these two isotopes in the right 
proportion to give an atomic weight of 207-2 ! 

Their spectra, for all practical purposes, are identical 
with one another and with that of ordinary lead. But, 
quite recently, a minute difference of wave-length has 
been established in the case of one of the brightest lines, 
a difference that does not exceed one part in ten million 
or one-thousandth of the difference between the two 
sodium lines Di and D2. It is so minute that it can 
only with difficulty be established by the most refined 
measurements. Nevertheless this difference between 
the spectra of isotopes is likely to prove of great 

Agreeably, however, with what is to be expected for 
isotopic atoms having identical shells but nuclei of 

^ The ingenious suggestion has been made that it might be used to 
separate the isotopes of chlorine (p. 248). A beam of Hght filtered 
through chlorine will lose first the vibrations corresponding with those 
of the lighter isotope, since it is in predominant quantity, and may 
then be able to stimulate the heavier isotope only to react with hydrogen, 
thus effecting the separation. This is being tried at Oxford, and at 
the time of writing (July, 1920) the results appear most promising 
(T. R Merton and H. B. Hartley, Nature, March 25, 1920). 


different mass, the densities of the different kinds of 
lead are different just in proportion to the differences in 
atomic weight. In other words, the different isotopic 
atoms have the same volume. 

Another precisely parallel case has been established 
for the isotopic elements ionium and thorium. We have 
seen (p. 153) that, on account of its period being forty 
times longer than that of radium, the amount of ionium 
in a mineral must be something like 12-5 grams per ton 
of uranium, or 58 grams per gram of radium. Now all 
uranium minerals yet examined on a sufficiently large 
scale contain, probably, a larger quantity of thorium 
than this. It is a suggestive and unexplained point 
that the proportion is smallest in the secondary recent 
uranium minerals. In practically all the primary 
uranium minerals several per cent, of thorium is found. 
Thus, ionium can never be obtained pure free from its 
isotope, thorium, but from a suitable secondary uranium 
mineral, a preparation containing a considerable pro- 
portion of ionium, admixed only with thorium, may be 
separated. Such a preparation separated from 30 tons 
of Joachimsthal pitchblende by Auer von Welsbach, has 
been investigated by Honigschmid. For pure ionium 
an atomic weight, 230, is to be expected, since it changes 
into radium with expulsion of an a-particle. The atomic 
weight of the ionium-thorium mixture described was 
found to be 231-51, whereas that of pure thorium, by 
the same method, was 232-12. But the spectrum of the 
thorium-ionium preparation was, so far as could be 
seen, identical with that of the pure thorium preparation, 
and in both no impurities whatever could be detected. 
The elimination of everything but ionium from the 
thorium by the elaborate chemical purifications adopted 
in the treatment of the material had been effected, but 
these methods are incapable of affecting, to the slightest 
degree, the ratio of the ionium to the thorium. 


Separation or Isotopes. 

It would be idle to deny that these new ideas, that 
different nuclei may exist in atoms which, to the chemist 
and spectroscopist, are indistinguishable and insepar- 
able, cuts far more deeply into the basis of chemical 
theory than did the discovery of the actual disintegra- 
tion of the radio-elements and of the spontaneous evolu- 
tion of one element from another. It is of interest to 
inquire into the possibilities of separating a mixture of 
isotopes, or, if this is impracticable, of detecting their 
separate existence in a mixture without separating 
them. It will be obvious that any property which 
involves directly the atomic mass could, theoretically, 
if not practically, be employed for their separation and 
separate detection. But it is remarkable how difficult 
such methods are to apply to this purpose, and how few 
of them ever have been used as practical aids to chemical 
analysis. The rate of diffusion of a gas, or, less suitably, 
of a substance dissolved in a liquid, depends directly on 
the molecular weight of the substance and therefore of 
the weight of the separate atoms the molecules contain. 
Theoretically, thorium and ionium, the two uraniums or 
common lead itself, if it is a mixture of isotopes as is 
possible, ought to be capable of resolution by diffusion 
methods. But this has not yet been practically achieved. 
Other methods, such as depend upon centrifuging the 
mixed material, or submitting it to the process of 
thermal diffusion, have been proposed but not yet 
successfully carried out. 

It would be an extraordinarily difficult and laborious 
piece of work, for example, to separate the constituents 
of the air in a pure state by diffusion, though a partial 
and incomplete separation by this means might easily 
be effected. It is not a method a chemist would 
employ unless he were obliged. On the other hand, 
though on the point there is a difference of opinion, any 
commonly used purely physical method other than those 


mentioned, such as fractional distillation, crystallisation, 
or adsorption, is not likely, even theoretically, to be 
effective in separating a mixture of isotopes. These 
certainly depend upon the chemical character of the 
element, rather than its atomic mass. 

Neon and Metaneon. 

Interesting, because it was discovered just at the 
time that the true interpretation of isotopes had been 
found, and also because it concerns an element very far 
removed from the heavy elements at the end of the 
periodic table undergoing radioactive change, is the case 
of neon and metaneon. The element neon is one of the 
inert gases, similar to argon, existing in the atmosphere 
to the extent of some twelve parts per million by volume. 
It is intermediate, in the zero family of elements, between 
helium with atomic weight 3-99 and argon with atomic 
weight 39-9, exactly ten times greater — both these being 
practically whole numbers. The atomic weight of neon 
is 20-2, a number differing from the nearest integer by 
a fifth of a unit. 

As a sequel to his classical work (p. 57) in elucidating 
the charge and mass of the electron, which constitutes 
the cathode ray of the vacuum tube, Sir Joseph Thomson 
applied similar methods to the positively charged par- 
ticles, or " positive rays " as they are called, which under 
certain circumstances can also be detected in the 
vacuum tube discharge. Here in every case so far 
examined the mass of the particle is never less than 
that of the hydrogen atom, and often it is much greater. 
In fact, so was developed a novel method of determining 
the atomic mass of elements, such as hydrogen, oxygen, 
nitrogen, and other gases which are present as positive 
ions in the vacuum tube discharge, and the molecular 
weight of such particles as so exist in groups of more 
than one atom. One of the most interesting of the 
numerous discoveries made was that of the gas called 
X3, which has a mass three times that of the hydrogen 


atom, and which is, in all probability, the molecule H3, 
analogous to ozone, the allotropic form of oxygen, O3, 
though chemists have never yet prepared or observed 
the existence of such an allotrope of hydrogen. ""■ But the 
same is true of many groups, such as CH, CH2, CH3, for 
which this new and exceedingly delicate method of gas 
analysis indicates at least a passing existence. 

The interest of this method, depending as it does 
directly upon the mass of the atom or molecule, from 
the present point of view is that, undoubtedly, it would 
be capable of revealing, if they existed, in any gaseous 
element, the separate individual components of a mix- 
ture of isotopes of different atomic mass. It is, in fact, 
almost the only practical method that could do so 
without ambiguity. Now, in examining the positive 
rays produced in neon by the electric discharge. Sir 
Joseph Thomson and Mr. Aston found in addition to the 
neon atom carrying a single positive charge, Ne+, of 
mass 20, a much fainter indication of another atom 
with a single + charge, of mass 22, which provisionally, 
as it could not be ascribed to a known element, they 
attributed to a new gas which they named metaneon. 

The question at once arose whether this was a case of 
the isotopism with which we have become familiar in 
the case of lead and the radio-elements. An attempt 
to separate neon and metaneon from ordinary neon, by 
a prolonged series of fractional absorptions of the gas 
in cooled charcoal, effected no separation whatever. 
The density of the fractions separated by the process 
were identical and the same as before the treatment, 
whereas metaneon, with atomic weight 22, should have 
a density 10 per cent, greater than neon with atomic 
mass 20. But this, as we have seen, is to be expected 
of isotopes, for in all probability the ordinary physical 
properties, such as volatility, etc., are, hke the chemical 

i This differs from the new particle of mass three more recently 
obtained by Rutherford in the bombardment of oxygen and nitrogen 
atoms by a-particies, in that it carries a single instead of a double 
positive charge. 


properties, indistinguishable. Neon remains still un- 
resolved into its two components, though after a long 
series of fractional diffusion experiments some in- 
dication of a partial separation was obtained. 

But the latest information confirms the existence of 
metaneon in the gas. Aston has developed the positive 
ray method of analysis considerably, so that it is capable 
of fixing with great precision the atomic or molecular 
weight of the particle causing the positive ray. His 
measurements showed neon to be a mixture of two gases 
of atomic weight 20-00 and 22-00 to within an error of 
one part in a thousand. So we may conclude with 
considerable probability that these two isotopic gases, 
in proportion of about 90 per cent, of the first and 
10 per cent, of the second, constitute the ordinary 
element neon derived- from the atmosphere. 

The General Prevalence of Isotopism. 

At the time of correcting the proofs of this book 
(July, 1920), this work of Aston has developed into one 
of the most important contributions of recent times to 
our knowledge of the chemical elements. The new 
methods, a brilliant outcome of combined mathematical 
and experimental ability, have proved themselves to be 
of extraordinary power and accuracy in the detection of 
isotopes and the measurement of their separate atomic 
weights. By altering the mode of application of the 
electric and magnetic deviating fields, an effect of the 
utmost practical service, analogous to the focussing effect 
of an ordinary lens on light, was secured, whereby all 
the particles of the same mass and charge in a narrow 
diverging cone of positive-rays are brought to a focus 
at a point, the foci for different particles lying on a 
straight line, in the plane of which the photograhhic 
plate is put. Each particle thus records its position 
as a spot or line on the plate, and there results an analysis 
of the beam into its different constituent particles, quite 
analogous to the resolution of light into constituent 



lines in a spectrum. From the position of the lines on 
the photographic plate, the mass of the atom producing 
it can be determined with an accuracy scarcely, if at all, 
inferior to that attained by chemical methods in the 
finest atomic weight determinations. But the method 
has the added inestimable advantage that mixtures of 
isotopes show their several atomic weights rather than 
the mean value, which is all that can be got from 
chemical determinations. 

The results of this new method so far announced are 
sufficiently startling. Eighteen elements have, as yet, 
been examined. Of these, nine only were found to be 
homogeneous. The other nine consist of mixtures of 
from two to as many as five or more isotopes. More- 
over, in every case, except hydrogen, the true atomic 
weight is found to be an exact integer (in terms of the 
atomic weight of oxygen as 16, taken as the standard 
of comparison) to an accuracy of one part in a thousand. 
For hydrogen, the atomic weight on this basis, 1-008, 
deduced by chemists from some of the finest atomic 
weight work ever performed, has been exactly con- 
firmed. The results are collected in the table below. 

" Pure " 


'■' Mixed ^^ 

Number of 











10-00 and 11-00 




Two , 

20-00 and 22-00 




Two or three 

28-0, 29-0, and 





36-0 and 40-0 





35-0 and 37-0 





79-0 and 81-0 

Phosphorus . . 



Five or six 

78 (?), 80, 82, 83, 
84, and 86 




Five (?) 

and 135 




Five or more 

202, 204, and 
three or four 
unresolved be- 
tween 197 and 

As shown by the intensities of the different lines, the 
proportion in which the isotopes are present accord 


well in each case with the value of the mean atomic 
weight as determined chemically. Thus the two isotopes 
of bromine are in similar proportion, but the lighter 
isotope of argon is barely detectable. It is thus not 
too much to suppose that all the atomic weights, except 
hydrogen, are exact integers, and that the fractional 
values found by chemists for some of the elements are 
due to their being mixtures of several isotopes. 

The Problem of Transmutation. 

From the picture we have formed of the general 
structure of the atom and the view we have of what 
exactly would constitute a transmutation, we may 
attempt, in conclusion, to consider the kind of methods 
by which its accomplishment might practically be 
attempted. It is clear that it is the nucleus of the atom 
that has to be changed, either by adding to or sub- 
tracting from it positive or negative charges. The sub- 
traction or addition of electrons, so far as the outermost 
shell of the atom is concerned, in no sense constitutes a 
transmutation, but is what occurs in ordinary chemical 
changes. In the free state of the element the atom is 
electrically neutral. The number of external electrons 
is equal to the net positive charge of the nucleus. 
Subtraction of one " valency " electron or more from 
the outermost shell produces the positive ion, which is 
characteristic, not of the free element, but of it when 
combined with other elements to form chemical com- 
pounds. But such additions and subtractions are con- 
fined to the outermost shell. There is no exchange yet 
capable of being effected between the electrons in the 
inner completed rings and either the electrons in the 
outermost ring or the electrons inside the nucleus. 
When, however, the nucleus spontaneously ejects posi- 
tive or negative charges, as it does in the a- and /3-ray 
changes, a complete and instantaneous rearrangement of 
the electrons both in the completed rings and the outer 
shell appears to follow. In brief, to transmute an atom, 


the change has to be effected from within, outwards 
from the central nucleus. It cannot, at least as yet, 
be impressed upon the nucleus by any changes in the 
exterior electronic shell, imposed from without. 

But the comparative ease with which the outer shell 
of the atom may be altered by chemical and also by 
electrical forces imposes in itself a formidable practical 
barrier to any more deep-seated change. 

We have seen that the a-particle may be regarded as 
the agent most likely to break up the nucleus of an 
atom if it impinges upon it, and that this actually may 
occur in the case of the nucleus of the nitrogen atom. 
Is it possible artificially to generate an a-particle or one 
possessing a similar amount of kinetic energy ? 

It may be calculated that the energy of the a-particle, 
over the range of velocity so far studied, is such as it 
would acquire in passing between two points differing 
in electric potential by from two to four million volts. 
This gives a quantitative idea of the strength of the 
electric field likely to be required before particles anal- 
ogous to the a-particle could be successfully produced. 

We may be fairly certain that the only influences 
likely to be effective in transmuting matter will be 
electrical in character, and that very much higher poten- 
tials at present known or utilised in electrical engineering 
will have to be developed before there is much chance of 
success. Along this road much that is new and impor- 
tant will first have to be made clear. So far as it has 
been followed, a barrier to further progress has been 
reached, which may or may not prove to be fundamental. 
The attainment of very high potentials at present seems 
to be limited by the failure of the insulation. Even a 
practically perfect vacuum, it appears, fails to insulate, 
and transmits a discharge across it when the potential 
exceeds a certain limit. 

Moseley hit upon the very ingenious idea of using the 
radium clock (Fig. 15, p. 59), as a method of arriving 
simply at otherwise unattainable potentials. If the 
clock there depicted is deprived of its leaves, if the 


insulating support of the radium can be made good 
enough and the vacuum sufficiently nearly perfect, there 
ought, theoretically, to be no limit to the extent the 
radium would become positively charged, and therefore 
to the difference of potential between it and the sur- 
rounding wall, unless, thereby, the radium products 
were prevented from further disintegrating and emitting 
their /3-rays. 

In practice Moseley could not, with his particular 
apparatus, attain a potential much above 150,000 volts. 
A discharge through the vacuum always occurred at this 

The reason probably is that the loosely held " va- 
lency " electrons in the outermost shell of the atoms 
constituting the surfaces are dragged out of the atom 
by the electric field so causing the discharge. Such a 
change is not transmutational, but is allied to or identical 
with that produced by ordinary chemical agencies. It 
indicates that there is a definite limit to the extent to 
which matter can be charged, and at present this rather 
closes the door to further progress. 

The outer regions of the atom effectively guard the 
inner from being attacked. If a perfect vacuum is 
unable to withstand the electric forces without trans- 
mitting the discharge, it may be expected that any 
material insulator is even less likely to do so. 


This must conclude the attempt to deal with the 
numerous and important advances made since these 
lectures were first given. The field of work has opened 
out in a number of directions previously unsuspected. 
The problem of transmutation and the liberation of 
atomic energy to carry on the labour of the world is no 
longer surro.unded with mystery and ignorance, but is 
daily being reduced to a form capable of exact quanti- 
tative reasoning. It may be that it will remain for ever 
unsolved. But we are advancing along the only road 


likely to bring success at a rate which makes it probable 
that one day will see its achievement. 

Should that day ever arrive, let no one be blind to the 
magnitude of the issues at stake, or suppose that such an 
acquisition to the physical resources of humanity can 
safely be entrusted to those who in the past have con- 
verted the blessings already conferred by science into a 
curse. A.S suddenly and unexpectedly as the discovery 
of radioactivity itself, at any moment some fortunate 
one among the little group of researchers engrossed 
in these inquiries might find the clue and follow it 
up. So would be diverted into the channels of human 
consciousness and purpose the full primary fountain of 
natural energy at its source, for use or misuse by men, 
according as to whether the long and bitter lessons of 
the painful past and present have even yet been really 



a-particles, Collision of, with 
matter, 62-67, 223, 224 

— Bombardment of gases with, 


— Coloration of mica and gems by, 


— Connection of, with helium, 44, 

60, 93-104, 

— Energy of, 61 

— from radium itself, 94 

— from the emanation, 79, 144 

— from uranimn, 149 

— Individual, 42, 44-46, 61 

— Limiting velocity of, 61, 66 

— Mass of, 60, 98 

— Niunber of, expelled by radium, 

40, 42, 45 

— passage through atoms, 220- 


— Positive charges carried by, 

60, 63 

— Proof of identity with helium 

of, 102 

— Scattering of, 63, 222 

— Tracks left by, 65 

— Velocity of, 61, 66, 94, 161, 

221, 249 
a-ray product, chemical properties 

of, 228 
a-rays, 41-67 

— Absorption of, 33 
by air, 34 

— Connection between range of, 

and period of substance, 164 

— Magnetic deflection of, 60 

— Making paths of visible, 64 

— Range of, 34, 45, 133, 161, 164 
in mica, 166 

— Resolution of, 41-46 
Accumulation of products, 95-98, 

Actinivun, disintegration series, 
186, 198-204, 207, 214. 229 

— Emanation, 198, 203 

— Origin of, 199, 204 

— Parent of, 205 

Actinium, period of life of, 198 

— Production of helium from, 

99, 102 

— A, 198, 203 

— B, C, and D, 198 

— X, 198 

Active deposit of actinium, 198- 

of thorium, 194-197 

of radium, 137-144 

Residual activity from, 


Age of the earth, 25, 75, 98, 

Ages, The geological and incan- 
descent, 179 

Alchemist, The problem of the, 

Alkaline-earth elements, 85, 105 

Alternative theories of radio- 
active energy, 68, 89 

Aluminium, 205, 213, 214 

— carbide, 213 

Analogies between the disinte- 
gration series, 188-191, 198 
Angstrom units, 237 
Antimony, 214 
Anionoff,G.N., 205 
Argon, '84, 85, 97, 105, 214, 240 

— atomic weight, 248 
Arrhenius, Svanie, 215 
Arsenic, 214 

— Atomic weight of, 248 
Aston, F. W., 246, 247 
Atom, Definition of, 105-108 

— Innermost region of, 220 

— Intermediate region of, 238 

— Model, 212 

— Nuclear, 220 

— Outermost region of, 217 

— Structure of, 210 

Atomic disintegration, 39, 58, 67, 
89, 94, 96, 98, 105, 109, 
112, 155, 157, 168, 209 

Cause of, 14 

Multiple, 200 

— mass or weight, 240, 248 





Atomic number, 231, 239, 240 

— property. Radioactivity an, 12, 

13, 15, 68, 74, 83, 110 

— synthesis, 180, 208 

Atoms, 2, 12, 40, 46, 60, 63, 66, 
84, 105, 158-161 

— Interpenetration of, 63 

— Passage through, of a-particles, 

Atoms, Solar systems compared 

and contrasted with, 226 
Autunite. 98, 148 
Average life, Determination of, 

of common elements, 157 

of emanation, 113 

of ionium, 134 

of radium, 117, 125, 207 

of thorium, 207 

of uranium, 116, 125, 207 

Period of, 112, 207 


/3-particles, 49-59 

— Charge of, 50, 51, 58 

— Mass of, 57 

— Tracks left by, 65 

— Velocity of, 58 

)3-ray product, Chemical proper- 
ties of, 228 
j8-rays, 29-66, 228, 250 

— Magnetic deflection of, 48, 60 

— Making paths of visible, 165 
/3-rays, Definition of, 139 
Barium, 15, 85, 214 

Barkla, C. G., 238 

Becquerel, Henri, 6, 7, 128 

Beryllium, 214 

Bismuth, 15, 146, 148, 191, 214, 

Boltwood, B., 125, 133 
Bonds of affinity, 213 
Boron, 205, 214 

— Atomic weight of, 248 

Bragg, Sir William, 34, 35, 45, 62, 

63, 64, 220, 235 
Branch Series, 201 
Breviiun, 150, 214 
Broeck, van der, 231 
Bromine, 214 

— Atomic weight of, 248 
Bunsen, K. W.,75 

7-rays, 29-32, 66, 237 
— Radiograph by, 31 
Cadmium, 214 
Caesium, 75, 214, 239 

Calcium, 214 

— absorption of gases by, 52, 101 
Carbon, 106, 213, 214, 224 

— Atomic weight of, 248 
Carnotite, 20, 98 
Cascade of changes, 74 
Cathode-rays, 52-58, 210, 245 
Cause of atomic disintegration, 114 
Cerium, 214 

Chance of disintegration. 111 
Change, Law of radioactive, 112 

— of radio-elements, 71, 74, 91, 

92 el seq. 
Chemical combination. Nature of, 

— elements, bonds of affinity, 213 
Number of, 239 

Order of, 212 

Table of, 214, 231, 239 

Chemists and radioactivity, 109 
Chlorine, 105, 213, 214, 215 

— atomic weight, 248 
Chlorion, 213 
Chromium, 214 

Cloud method of making paths of 

rays visible, 64 
Cobalt, 214, 240 

Conservation of radioactivity, 88 
Constancy of radioactivity, 10, 13, 

23, 24, 27, 43, 69, 70, 77, 90, 172 
Control of natural energy, 5, 13, 

173, 184 
Copper, 214 
Corpuscular theory of radiation, 

Cosmical aspect of life, 179 

— energy, 24, 120, 174, 178 
Cost of scientific investigations, 19 
Cranston, J. A., 204 

Crookes, Sir William, 15, 42, 52, 

57, 128, 165 
Crookes' tubes, 52, 58, 210 
Crystal, space-lattice, 235 
Curie, M. and Mme., 10, 12, 13, 

15,19, 75,83, 124, 136,145, 198 


" Dg" line, 97, 100, 101 
Dalton, John, 106, 108, 217 
Debierne, A., 99, 198, 203 
Decay of radioactivity, 70, 87 
Definition of the atom, 105-108 
Detection of infinitesimal quan- 
tities, 17, 75, 77, 82, 85, 90, 91, 
95, 109 
Determination of average life, 115 
Dewar, Sir James, 52 
Diffraction grating, 234 



Discovery of radioactivity, 6 
Discrete theory of radium rays, 

40, 44 
Disintegration, see Atomic dis- 
— , Chance of, 111 

— series, Analogies between, 188- 

191, 198 

of actinium, 186, 198-204 

of thorium, 178, 186-198 

of uranium, 121-151 

Doctrine of energy, 20, 27, 37, 

68, 178, 185 
Dysprosium, 214 


" E-ray," 204 

Earth, Age of the, 25, 75, 177-183 

— Internal heat of, 178 
Earthquake routes, 180 
Effects of radioactivity, 8-11, 28 
Eka-tantalum or proto-actinium, 

Electric current, Action of magnet 

on, 49 
Electricity, Discharge of, 8, 14, 18, 
34, 45, 64, 211 

— Nature of, 50 
Electrolytic dissociation. Theory 

of, 213-217 
Electro-magnet, 47 
Electro -magnetic inertia, 211 
Electrometer, 46 
Electron theory of matter, 109, 

Electrons, 55-58, 63, 109, 210, 212, 
216, 247 

— period of revolution, 236 

— valency, 217, 249 
Electroscope, Gold-leaf, 8, 17, 42, 

59, 84 
Electrostatic and electromagnetic 

deflection methods, 55, 57, 60, 

225, 245, 247 
Elements, Chemical, bonds of 
affinity, 213 

Number of, 239 

Order of, 212 

Stability of, 72, 157, 163 

Table of, 214, 231, 239 

Unchanging character of, 72, 

73, 163, 227 

— Isotopic, 229, 231 

— Rare-earth, 218 

— Rarity of, 155 
Elixir of life, 182 

Emanation of radium, 68, 77-94, 

— a-particles from, 79, 145 

Emanation of radium. Atomic 
weight of, 85, 103 

— Average life of, 113, 116, 122 

— Chemical nature of, 84, 105 

— Condensation of, 80-82 

— Density of, 85 

— Heat generated by, 85, 86, 170 

— Physiological action of, 82 

— Rate of decay of, 87, 88 

— Reproduction of, 88, 89, 90j 


— Spectrum of, 85 

— Volume of, 82, 119 

— of actinium, 198, 203 

— of thorium, 136, 190, 193-198 
Emanations and radiations con- 
trasted, 78 

Emanium, 203 

Energy, cosmical. Source of, 174 

— Doctrine of, 20, 27, 37, 68, 178, 


— Internal, of matter, 68, 71-73, 

86, 87, 91, 96, 108, 168-176 

— Measurement of, 22, 71 
Energy of coal, 22, 23, 70, 120 

— of radioactive substances, 3, 5, 

10, 58, 62, 68, 91, 92, 172 

— of radium, 22, 68, 86, 119, 171 

— of uranium, 170-172 

— Transformers of, 69, 90 
Ephemeral transition-forms, 74, 

92, 116, 121, 129, 203 
Equilibrium, Radioactive, 90, 95, 

117, 196 
Ether, The, 37, 38, 56 
Erbium, 214 
Europiiun, 214 

Evolution of elements, 134, 162, 

— of universe, 26, 120, 175 
Existence, Struggle for, 6, 184 


Facts and theories of radio- 
activity, 89, 108 

Fajans, K., 229 

Faraday, Michael, 47, 55 

Fleck, Alexander, 228 

Fletcher, A. L., 166 

Fluorescence, 6, 18, 31, 53, 66, 
78, 79, 195 

Fluorine, 214 

— Atomic weight of, 248 
Friedrich, M., 235 


Gadolinium, 214 
Gallium, 205, 214 



Gas, A radioactive, 77, 80, 83 

Gases, bombarded by a-particles, 

Geiger, Dr., 45 

Geological bearing of radioac- 
tivity, 26, 75, 175-180 

Geology, Controversy between 
physics and, 26 

Germanium, 205, 214 

Giesel, F. 0., 20, 99, 203, 204 

Gold, 214, 231, 233, 239 

— currency, 156 


H-particles, 223, 224, 225 
Hahn, Otto, 188, 205 
Halogen family, 218 
Halos, Pleochroic, 165 
Hartley, H. B., 242 
Heat generated by radium, 18, 19, 
22, 85, 119, 178 

in the earth, 178 

Heaviside, Oliver, 210 
Helium, 44, 60, 84, 94-104, 209, 

— atomic number, 231 
weight, 245, 248 

— Discovery of, 97 

— Liquefaction of, 97 

— Possible isotope of, 225 

— Prediction concerning the 

origin of, 98 

— Production of, by radium, 94, 


by actinium, 99, 102 

by thorium and uranium, 100 

— in radioactive minerals, 96, 97 
Volume of, 98 

— Spectrum of, 99 
Hersch3ll, Sir John, 159, 162 
High vacua, 50, 52 
Hitchins, Miss A. F., 130, 134 
Holmium, 214 
Homogeneous Characteristic X- 

rays, 238 
HSnigschmid, O., 243 
Huggins, Sir William, 107 
Hydrogen, 107, 214, 224, 289 

— atomic number, 231 
weight, 248 

Incandescent age, 179 

— gas-mantle, 14, 187 

Increase of activity of radixim 
with time, 16, 155 

Indifference of radium to its en- 
vironment, 27, 77 

Indium, 214 

" Induced radioactivity," 136 

Inertia, 56, 211 

Infinitesimal quantities. Detection 

of, 75, 76, 82, 85, 90, 91, 95, 109 
Inglis,J. K.H., 113 
Integral values of atomic weights, 

Intermediate substances, 74, 76, 

77, 131-135 
Internal energy of matter, 68, 70, 

71-73, 86, 87, 168 

— heat of earth, 178 
Interpenetration of atoms, 63 
Iodine, 214, 239 

lonisation of gases, 8, 63, 64, 66 

— of liquids, 215 

Ionium, 133, 151, 153, 154, 164, 
189, 205 

— atomic weight, 243 

— Average life of, 134 

— Estimated period of, 134, 165 

— and uranium X, Connection 

between, 133 
Iridium, 214, 239 
Iron, 106, 214 
Isotopes, 133, 150, 160, 229, 231- 

233, 240, 248 

— Separation of, 243, 248 

Joachimsthal mine, 15, 152 

Joly, John, 166, 176-178 

— "Radioactivity and geology, 



K-Series of X-rays, 238 
Kalgurli, mines at, 132 
Katrine, Loch, 125, 131 
Kelvin, Lord, 20, 37, 178 
Kirchoff, 75 
Knipping, P., 235 
Krypton, 214 

L-Series of X-rays, 238 
Lanthanum, 198, 214, 218 
Laue, M., 235 

Law of proportionality, 118, 123, 

— of radioactive change. 111 
Lead, 214, 231 

— Atomic weight of, 241 

Lead and radiiun, Connection be- 
tween, 15, 76, 148, 241 

— and thorium. Connection be- 
tween, 191, 241 

Life from the cosmical standpoint, 



Life of radio-elements, 92 

— Period of average, 113 
Light, Nature of, 36, 39 

— Velocity of, 38, 58, 211 
Limitations of knowledge, 4, 6, 

66, 173, 178-180, 227 
Lithium, 214 

— atomic number, 231 
Lutecium, 214 


M-Series of X-rays, 238 

Macdonald laboratories of M'Gill 
University, 89 

Mackenzie, T. D.,130 

Magnesium, 214 

Magnetic deflection of cathode- 
rays, 53 

Maintenance of radium, 121 

sun's energy, 24, 120, 179 

Manganese, 214, 239 

Marckwald, W., 146, 147, 192 

Marsden, E., 223 

Mass of the electron, 55-57, 210 

Matter, Electron theory of, 109 

— Ultimate structure of, 209 

— Unsolved problems of, 109, 

Maxwell, J. Clerk, 158, 162, 181 
McCoy, H. N., 125 
Measurement of energy, 22, 71 
Meitner, Miss L., 205 
Mendelejeff, D., 205 
Mental pictures, 109 
Mercury, 85, 148, 214, 231 

— atomic weight, 247 
Merion, T. R., 242 
Mesothoriima, 187-193, 227 
Metaneon, 244 

— atomic weight, 248 

Mica, Coloration of by a-rays, 166 
Milngavie, reservoir at, 125, 131 
Minerals, Helium, in radioactive 

— Lead in radioactive, 15, 148 

— Quantity of radium in, 16, 75, 

123, 152 

— Ratio between quantities of 

uraniiun and its products in, 

Minimum quantity of helium de- 
tectable, 101 

radium detectable, 17, 42 

Molecules, 2, 108, 158 
Molybdenum, 214 
Monazite sand, 186, 192 
Moseley, H. G. J., 239, 249 
Multiple atomic disintegration, 200 


N-particles, 224, 225 

Negative and positive electricity, 

Neodymium, 214 
Neon, 84, 214, 245 

— atomic weight, 245, 248 
Newton, Sir Isaac, 38 
Nickel, 214, 240 
Niobium, 214 
Nitrogen, 213, 214, 224 

— Atomic weight, 248 
"Niton," 78 

Nomenclature concerning atoms 
and molecules, 106-108 

Non - separable radio - elements, 
154, 187, 195, 227 

Nuclear atom, 61, 210, 220 


O-partieles, 224, 225 
Onnes, K., 97 
Osmium, 214, 239 
Ouroboros, 181 
Oxygen, 106, 214, 224 

— Atomic weight, 243 

P-3 route of earthquakes, 180 
Palladium, 214, 239 
Parent of ionium, 133 

— of radium, 122-134 
Penetration test of rays, 7, 29, 30, 

31, 80 
Period of average life, 113 

connection with range 

of a-rays, 164 

— half change, 115 

Periodic law, 105, 171, 205, 212, 

— table of the chemical elements, 

214, 228, 231, 239 
Perpetual motion, 21, 24, 59 
Phosphorescence, see Fluores- 
Phosphorus, 214 

— Atomic weight of, 248 
Photographic effects of radio- 
activity, 8, 14, 18, 66, 80 

Physical impossibility, 25 
Pitchblende, 15, 75, 127, 152, 

187, 243 
Planet, niunber of revolutions, 236 
Platino-cyanides, 31, 35, 141 
Platinum, 214, 239 
Pleochroic halos, 165 
Polonium, 16,44, 146, 154,199,214 



Positive and negative electricity, 

— rays, 245, 246 
Potassium, 105, 214, 240 
Praseodymium, 214 
Prediction of origin of helium, 

Proportionality, Law of, 118, 123, 

Proto-actinium, 205 


Quantity of helium detectable by 

spectroscope, 101 
in minerals, 98 

— of radium in minerals, 16, 75, 
124-127, 152 


Radiant matter, 52, 57, 211 
Radiation, Nature of, 36-39 
Radiations, Complex, 28 
Radioactivity, a new science, 1 

— discovery, 6, 26, 175 

— Four experimental effects of, 8 

— an unalterable atomic pro- 
perty, 12 

Radiograph by 7-rays, 31 
Radio-tellurium, 146, 148 
Radio-thorium, 187-196, 227 
Radivun and uranium, connection 
between, 124-127 

— Active deposit of, 137 

— Average life of, 117, 125 

— Changes of, 136 

— Chemical nature of, 15 

— clock, 59, 249 

— Cost of, 19 

— A changing element, 73 

— emanation. See Emanation of 


— Experiments with, 18 

— Growth of, 134 

— Maintenance of, 121-135 

— " physically impossible," 26 

— Quantity of, in pitchblende, 


— Radiations from, 139 

— Reproduction of, 122 

— series, 207 

— Substitute for, 154, 187, 193 

— War uses of, 19 

Radium A, 89, 105, 139-144, 153 

— B, 89, 105, 139-144 

— C, 105, 139-144, 151, 164, 201 

— C, 151, 202 

— D, 153, 190 

— D, E, and F, 145-147 

Radium F, Identity of, with 

polonium, 147 
Ramsay, Sir William, 78, 82, 84, 

85, 97, 99, 119, 188, 223 
Ratio between uranium and its 

products, 152 
Rayleigh, Lord, 59, 84 
Rays of radioactive substances, 9, 

28 et seq. 
Recoil, Radioactive, 103, 104 
Recovery of radioactivity of 

radiiun, 77, 88 
Rhodium, 214, 239 
Ronigen, Wilhelm K., discovery of 

X-rays, 6 
Rowland diffraction grating, 234 
Rowland, Professor, 160 
Royds, T., 102 
Rubidium, 214 
Russell, A. S., 229 
Ruthenimn, 214, 239 
Rutherford, Sir Ernest, 29, 30, 45, 

46, 60, 78, 80, 86, 89, 98, 102, 

119, 125, 136, 161, 188, 197, 220, 

222, 223, 224, 225, 231 

Samarium, 214 

Scandimn, 205, 214 

Scattering of a-particles, 63, 222 

Schuster, Arthur, 161 

Selenium, 214 

Self-induction, 211 

Sidot's hexagonal blende, 36 

Silicon, 205, 214 

— Atomic weight of, 248 

Silk tassel experiment, 18, 34 

Silver, 214, 239 

Simplon Tunnel, Radium in rocks 
of, 177 

Sodion, 215 

Sodium, 214, 215, 238 

Solar systems, compared and con- 
trasted with atoms, 226 

Spectra of isotopes, 242 

Spectroscope, 75, 91, 92, 97, 99, 
101, 129, 160, 163, 179, 209 

Spinthariscope, 42, 44, 65 

Stability of elements, 72, 157, 163 

Standard, The International 
radium, 17 

Strontivun, 214 

Struggle for existence, 6, 184, 

Strutt, Hon. R. J. (now Lord 
Rayleigh), 59, 125, 176 

Substitute for radium, 154, 187, 193 

Successive changes of radio ele- 
ments, 74, 77, 89, 110, 116, 
129-133, 138, 145-149 



Sulphur, 214 

— Atomic weight of, 248 

Sun's energy. Maintenance of, 24, 

120, 178-180 
Synthesis of atoms, 180, 204 

Table of atomic weights of " pure " 
and " mixed " elements 
(Aston), 248 

disintegration series com- 
plete, 207 

periods and quantities, uran- 
ium series, 153 

velocities and ranges of 

a-rays, m-anium series, 

— Periodic, of the elements, 214 
Chart showing sequence of 

a- and j3-changes through, 
Tantalum, 205, 214, 219 
Tait, Professor, Recent Advances 
in Physical Science, 20, 25, 26 
Tellurium, 214, 240 
Terbium, 214 

Thallium, 148, 214, 231, 233 
Theories and facts of radio- 
activity, 89, 108 
Thomson, Sir Joseph, 55, 57, 210, 

212, 245, 246 
Thorium, 13, 94, 97, 98, 100, 
102, 133, 136, 154, 186-198, 

— Active deposit of, 194-198 

— atomic weight, 243 

— halos, 166 

— disintegration series, 178, 186- 

198, 207, 227, 229 

— Production of helirnn from, 


— Ultimate product of, 190, 242 
Thorium A, 190, 197 

— B, C, D, 164, 190, 201 

— C, 201 

— Emanation, 190, 193-196 

— X, 190, 195 
Thulium, 214 
Tin, 214, 238 
Titanimn, 214, 218 

Total energy in radium, 119 

in uranium, 170-172 

Transcendental character of radio- 
activity, 27, 58 
Transformers of energy, 69 
Transmutation, 13, 71, 72, 172, 
182, 209, 223-225, 233, 248-250 
Tungsten, 214 


Ultimate product of thorium, 190, 

— products of radium, 76, 96, 123, 

147, 148, 242 
Ultra-material velocities, 63, 221 
Unchanging character of elements, 

72, 73, 163 
Unsolved problem of matter, 109, 

Uranium, 7, 12, 97, 98, 102, 107, 
116, 124, 148, 169-172, 188, 
189, 194, 207, 214, 229, 239 

— atomic nrnnber, 214, 231 

— Average life of, 116, 125 

— halos, 166 

— Production of helium from, 


— and radium. Connection be- 

tween, 124-134 

— I and II, 149, 165, 189, 195 

206, 227 

— Y, 205 

— X, 128-131, 133, 150, 188, 206, 


— XiandXg, 150 

and ionium. Connection be- 
tween, 133 

Vacua, High, 50, 52, 249 
Valency electrons, 217, 249 
Value of gold, physical explanation 
to accoiuit for the unchang- 
ing, 156 

— of radium, 19, 156 
Vanadium, 214 
Velocities, Ultra-material, 63 
Velocity of cathode-ray particle, 


— of light, 38, 58, 211 

Visible, Making the paths of rays, 

Volume of helium in minerals, 

— emanation in equilibrium with 

radium, 82, 119 


Wave-length of 7-rays, 237 
— of X-rays, 234-238 
Wave theory of light, 39 
Welsbach, Auer von, 13, 243 
Whytlaw-Gray, R., 85 
Willemite, 35, 53, 78, 79, 81, 

260 INDEX 

Wilson, C. T. B., 64 ♦ 
Writing by radium, 19 


X-rays, 6, 30, 31, 38, 78, 210, 234 

— Diffraction of, 234 

— wave-length, 234-238 
Xg gas, 245 

Xenon, 214 

— Atomic weight of, 248 

Ytterbium, 214 
Yttrium, 214 

Zero family, 217, 245 

Zinc, 214, 238 

— sulphide, 35, 80, 

197, 203, 204, 223 
Zirconium, 214, 239 

141, 196 


Back End Papers.